diff --git a/web-client/src/components/SrAdvOptAccordion.vue b/web-client/src/components/SrAdvOptAccordion.vue
index 3cebd031..1a24b225 100644
--- a/web-client/src/components/SrAdvOptAccordion.vue
+++ b/web-client/src/components/SrAdvOptAccordion.vue
@@ -3,9 +3,9 @@ import Accordion from 'primevue/accordion'
 import AccordionPanel from 'primevue/accordionpanel'
 import AccordionHeader from 'primevue/accordionheader'
 import AccordionContent from 'primevue/accordioncontent'
-import { defineAsyncComponent, ref } from 'vue'
+import { defineAsyncComponent, ref, computed } from 'vue'
 import { useReqParamsStore } from '@/stores/reqParamsStore'
-import { computed } from 'vue'
+import { photonProcessingAPIs } from '@/types/SrStaticOptions'
 
 /* eslint-disable @typescript-eslint/promise-function-async */
 const SrYAPC = defineAsyncComponent(() => import('@/components/SrYAPC.vue'))
@@ -39,6 +39,33 @@ const reqParamsStore = useReqParamsStore()
 const props = defineProps<Props>()
 const expandedPanels = ref<number[]>([])
 
+const isPhotonProcessingAPI = computed(() => photonProcessingAPIs.has(props.iceSat2SelectedAPI))
+
+// Fixed segment dimensions baked into pre-computed ATL06/ATL08 data products.
+// Source: ICESat-2 ATL06 / ATL08 product specifications.
+const fixedExtentsByAPI: Record<string, { len: number; res: number }> = {
+  atl06x: { len: 40, res: 20 },
+  atl08x: { len: 100, res: 100 }
+}
+
+const fixedExtents = computed(() => fixedExtentsByAPI[props.iceSat2SelectedAPI] ?? null)
+
+const fixedExtentsTooltip = computed(() => {
+  const api = props.iceSat2SelectedAPI
+  return `Segment dimensions are fixed in the ${api} data product and cannot be modified.`
+})
+
+// Panels that show parameters used at request time are visible when the
+// selected endpoint actually consumes them. X-series endpoints read pre-computed
+// segments from the source HDF5 product so most photon-processing panels are hidden.
+const showExtentsPanel = computed(() => isPhotonProcessingAPI.value || fixedExtents.value !== null)
+const showSurfaceElevationPanel = computed(() =>
+  ['atl06p', 'atl06sp', 'atl03x-surface'].includes(props.iceSat2SelectedAPI)
+)
+const showPhoRealPanel = computed(() =>
+  ['atl08p', 'atl03x-phoreal'].includes(props.iceSat2SelectedAPI)
+)
+
 const onPanelOpen = (value: any) => {
   //console.log('onPanelOpen', value, "type: ", typeof value);
   expandedPanels.value.push(value.index)
@@ -86,7 +113,7 @@ const fieldsHeader = computed(() => {
             <SrGranuleSelection />
           </AccordionContent>
         </AccordionPanel>
-        <AccordionPanel value="3" v-if="mission === 'ICESat-2'">
+        <AccordionPanel value="3" v-if="mission === 'ICESat-2' && isPhotonProcessingAPI">
           <AccordionHeader>Photon Selection</AccordionHeader>
           <AccordionContent v-if="isExpanded(3)">
             <SrAtl03Classification />
@@ -94,33 +121,37 @@ const fieldsHeader = computed(() => {
             <SrYAPC />
           </AccordionContent>
         </AccordionPanel>
-        <AccordionPanel value="4" v-if="mission === 'ICESat-2'">
+        <AccordionPanel value="4" v-if="mission === 'ICESat-2' && showExtentsPanel">
           <AccordionHeader>Extents</AccordionHeader>
           <AccordionContent v-if="isExpanded(4)">
-            <SrExtents />
+            <SrExtents v-if="isPhotonProcessingAPI" />
+            <div
+              v-else-if="fixedExtents"
+              class="sr-fixed-extents"
+              v-tooltip.top="fixedExtentsTooltip"
+            >
+              <div class="sr-fixed-extents-header">
+                <i class="pi pi-lock"></i>
+                <span>Fixed in {{ iceSat2SelectedAPI }} data product</span>
+              </div>
+              <div class="sr-fixed-extents-row">
+                <span class="sr-fixed-extents-label">Length</span>
+                <span class="sr-fixed-extents-value">{{ fixedExtents.len }} m</span>
+              </div>
+              <div class="sr-fixed-extents-row">
+                <span class="sr-fixed-extents-label">Step Size</span>
+                <span class="sr-fixed-extents-value">{{ fixedExtents.res }} m</span>
+              </div>
+            </div>
           </AccordionContent>
         </AccordionPanel>
-        <AccordionPanel
-          value="5"
-          v-if="
-            mission === 'ICESat-2' &&
-            (props.iceSat2SelectedAPI.includes('atl06') ||
-              props.iceSat2SelectedAPI.includes('atl03x-surface'))
-          "
-        >
+        <AccordionPanel value="5" v-if="mission === 'ICESat-2' && showSurfaceElevationPanel">
           <AccordionHeader>Surface Elevation</AccordionHeader>
           <AccordionContent v-if="isExpanded(5)">
             <SrSurfaceElevation />
           </AccordionContent>
         </AccordionPanel>
-        <AccordionPanel
-          value="6"
-          v-if="
-            mission === 'ICESat-2' &&
-            (props.iceSat2SelectedAPI.includes('atl08') ||
-              props.iceSat2SelectedAPI.includes('atl03x-phoreal'))
-          "
-        >
+        <AccordionPanel value="6" v-if="mission === 'ICESat-2' && showPhoRealPanel">
           <AccordionHeader>PhoREAL Veg Density Alg</AccordionHeader>
           <AccordionContent v-if="isExpanded(6)">
             <SrVegDensity />
@@ -331,4 +362,40 @@ const fieldsHeader = computed(() => {
   justify-content: center;
   margin-bottom: 0.5rem;
 }
+
+.sr-fixed-extents {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  padding: 0.75rem;
+  border: 1px solid var(--p-surface-300);
+  border-radius: var(--p-border-radius);
+  cursor: help;
+}
+
+.sr-fixed-extents-header {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  font-size: small;
+  color: var(--p-text-muted-color);
+  margin-bottom: 0.25rem;
+}
+
+.sr-fixed-extents-row {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 0.25rem 0.5rem;
+  opacity: 0.7;
+}
+
+.sr-fixed-extents-label {
+  font-size: large;
+}
+
+.sr-fixed-extents-value {
+  font-family: monospace;
+  font-size: large;
+}
 </style>
diff --git a/web-client/src/stores/reqParamsStore.ts b/web-client/src/stores/reqParamsStore.ts
index a129dd8a..5c9b2491 100644
--- a/web-client/src/stores/reqParamsStore.ts
+++ b/web-client/src/stores/reqParamsStore.ts
@@ -29,7 +29,8 @@ import {
   OnOffOptions,
   geoLocationOptions,
   signalConfidenceNumberOptions,
-  qualityPHOptions
+  qualityPHOptions,
+  photonProcessingAPIs
 } from '@/types/SrStaticOptions'
 
 export function getDefaultReqParamsState(): SrReqParamsState {
@@ -490,6 +491,8 @@ const createReqParamsStore = (id: string) =>
       getAtlReqParams(req_id: number): AtlReqParams {
         //console.log('getAtlReqParams req_id:', req_id);
         const req: AtlReqParams = {}
+        const isPhotonAPI =
+          this.missionValue === 'ICESat-2' && photonProcessingAPIs.has(this.iceSat2SelectedAPI)
         if (this.missionValue === 'ICESat-2') {
           if (!this.iceSat2SelectedAPI.includes('x')) {
             // atlnnx does not need asset set
@@ -561,7 +564,7 @@ const createReqParamsStore = (id: string) =>
             req.atl24.anc_fields = this.atl24AncillaryFields
           }
         } else {
-          if (this.missionValue === 'ICESat-2') {
+          if (isPhotonAPI) {
             if (this.enableAtl03Classification) {
               if (
                 this.surfaceReferenceType.length === 1 &&
@@ -625,7 +628,7 @@ const createReqParamsStore = (id: string) =>
           }
         }
 
-        if (this.missionValue === 'ICESat-2') {
+        if (isPhotonAPI) {
           if (this.getUseSurfaceFitAlgorithm()) {
             req.fit = {} as SrSurfaceFit
             if (this.getUseMaxIterations()) {
@@ -707,14 +710,16 @@ const createReqParamsStore = (id: string) =>
             }
           }
         }
-        if (this.passInvalid) {
-          req.pass_invalid = true
-        } else {
-          if (this.getUseAlongTrackSpread()) {
-            req.ats = this.alongTrackSpread
-          }
-          if (this.getUseMinimumPhotonCount()) {
-            req.cnt = this.minimumPhotonCount
+        if (isPhotonAPI) {
+          if (this.passInvalid) {
+            req.pass_invalid = true
+          } else {
+            if (this.getUseAlongTrackSpread()) {
+              req.ats = this.alongTrackSpread
+            }
+            if (this.getUseMinimumPhotonCount()) {
+              req.cnt = this.minimumPhotonCount
+            }
           }
         }
 
@@ -769,20 +774,20 @@ const createReqParamsStore = (id: string) =>
           // ATL03 classification settings would go here
         }
 
-        if (this.enableAtl08Classification) {
+        if (isPhotonAPI && this.enableAtl08Classification) {
           if (this.atl08LandType.length > 0) {
             req.atl08_class = this.atl08LandType
           }
         }
 
-        if (this.enableAtl24Classification) {
+        if (isPhotonAPI && this.enableAtl24Classification) {
           if (!req.atl24) req.atl24 = {}
           if (this.atl24_class_ph.length > 0) {
             req.atl24.class_ph = this.atl24_class_ph
           }
         }
 
-        if (this.enableYAPC) {
+        if (isPhotonAPI && this.enableYAPC) {
           let yapc = {} as Icesat2ConfigYapc
           yapc.version = this.getYAPCVersion()
           yapc.score = this.YAPCScore
@@ -798,7 +803,7 @@ const createReqParamsStore = (id: string) =>
           req.yapc = yapc
           //console.log('using req.yapc:',req.yapc)
         }
-        if (this.distanceIn.value === 'segments') {
+        if (isPhotonAPI && this.distanceIn.value === 'segments') {
           req.dist_in_seg = true
         }
         if (this.useServerTimeout) {
diff --git a/web-client/src/types/SrStaticOptions.ts b/web-client/src/types/SrStaticOptions.ts
index bf4d3406..33c0a6ac 100644
--- a/web-client/src/types/SrStaticOptions.ts
+++ b/web-client/src/types/SrStaticOptions.ts
@@ -26,6 +26,19 @@ export const iceSat2APIsItems = [
   'atl13x'
 ]
 export const gediAPIsItems = ['gedi01bp', 'gedi02ap', 'gedi04ap']
+// ICESat-2 endpoints that compute segments from ATL03 photons and accept
+// photon-processing parameters (cnf, srt, yapc, len, res, fit, etc.).
+// X-series endpoints (atl06x, atl08x, atl24x, atl13x) read pre-computed
+// segments from HDF5 files and ignore these parameters.
+export const photonProcessingAPIs: ReadonlySet<string> = new Set([
+  'atl03x',
+  'atl03x-surface',
+  'atl03x-phoreal',
+  'atl03vp',
+  'atl06p',
+  'atl06sp',
+  'atl08p'
+])
 export const surfaceReferenceTypeOptions = [
   { name: 'Dynamic', value: -1 },
   { name: 'Land', value: 0 },
diff --git a/web-client/tests/unit/endpointParamScoping.spec.ts b/web-client/tests/unit/endpointParamScoping.spec.ts
new file mode 100644
index 00000000..3b7256ed
--- /dev/null
+++ b/web-client/tests/unit/endpointParamScoping.spec.ts
@@ -0,0 +1,193 @@
+/**
+ * Unit tests for endpoint parameter scoping (Issue #1074)
+ *
+ * Verifies that X-series endpoints (atl06x, atl08x, atl24x, atl13x) do not
+ * include photon-processing parameters in their requests, since they read
+ * pre-computed segments from HDF5 files and the server ignores these params.
+ *
+ * P-series endpoints (atl06p, atl03x, etc.) compute from ATL03 photons and
+ * should include these parameters when enabled.
+ */
+
+import { describe, it, expect, beforeEach } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useReqParamsStore } from '@/stores/reqParamsStore'
+import { photonProcessingAPIs } from '@/types/SrStaticOptions'
+
+describe('Endpoint Parameter Scoping', () => {
+  let store: ReturnType<typeof useReqParamsStore>
+
+  beforeEach(() => {
+    setActivePinia(createPinia())
+    store = useReqParamsStore()
+    store.reset()
+  })
+
+  function enableAllPhotonParams() {
+    store.missionValue = 'ICESat-2'
+    store.enableAtl03Classification = true
+    store.enableAtl08Classification = true
+    store.atl08LandType = ['atl08_ground']
+    store.enableYAPC = true
+    store.YAPCScore = 100
+    store.useLength = true
+    store.lengthValue = 40
+    store.useStep = true
+    store.stepValue = 20
+    store.useSurfaceFitAlgorithm = true
+    store.useMaxIterations = true
+    store.maxIterations = 6
+    store.passInvalid = true
+    store.distanceIn = { name: 'segments', value: 'segments' }
+    store.useAlongTrackSpread = true
+    store.alongTrackSpread = 10
+    store.useMinimumPhotonCount = true
+    store.minimumPhotonCount = 5
+  }
+
+  const photonParamKeys = [
+    'cnf',
+    'srt',
+    'quality_ph',
+    'atl08_class',
+    'yapc',
+    'len',
+    'res',
+    'fit',
+    'pass_invalid',
+    'dist_in_seg',
+    'ats',
+    'cnt'
+  ]
+
+  describe('X-series endpoints should exclude photon-processing params', () => {
+    const xSeriesAPIs = ['atl06x', 'atl08x', 'atl24x', 'atl13x']
+
+    xSeriesAPIs.forEach((api) => {
+      it(`${api} request should not include photon-processing parameters`, () => {
+        enableAllPhotonParams()
+        store.iceSat2SelectedAPI = api
+        const req = store.getAtlReqParams(1)
+
+        for (const key of photonParamKeys) {
+          expect(req).not.toHaveProperty(key)
+        }
+      })
+    })
+  })
+
+  describe('P-series endpoints should include photon-processing params when enabled', () => {
+    it('atl06p request should include fit, len, res when enabled', () => {
+      enableAllPhotonParams()
+      store.iceSat2SelectedAPI = 'atl06p'
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('fit')
+      expect(req).toHaveProperty('len', 40)
+      expect(req).toHaveProperty('res', 20)
+      expect(req).toHaveProperty('pass_invalid', true)
+      expect(req).toHaveProperty('dist_in_seg', true)
+      expect(req).toHaveProperty('yapc')
+      expect(req).toHaveProperty('atl08_class')
+    })
+
+    it('atl03x request should include photon params when enabled', () => {
+      enableAllPhotonParams()
+      store.iceSat2SelectedAPI = 'atl03x'
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('len', 40)
+      expect(req).toHaveProperty('res', 20)
+      expect(req).toHaveProperty('pass_invalid', true)
+      expect(req).toHaveProperty('yapc')
+      expect(req).toHaveProperty('atl08_class')
+    })
+
+    it('atl08p request should include photon params when enabled', () => {
+      enableAllPhotonParams()
+      store.iceSat2SelectedAPI = 'atl08p'
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('len', 40)
+      expect(req).toHaveProperty('res', 20)
+      expect(req).toHaveProperty('pass_invalid', true)
+      expect(req).toHaveProperty('yapc')
+      expect(req).toHaveProperty('atl08_class')
+    })
+  })
+
+  describe('Endpoint-specific params still work for X-series', () => {
+    it('atl24x should include atl24-specific params', () => {
+      store.missionValue = 'ICESat-2'
+      store.iceSat2SelectedAPI = 'atl24x'
+      store.enableAtl24Classification = true
+      store.useAtl24Compact = true
+      store.atl24Compact = true
+      store.useAtl24Classification = true
+      store.atl24_class_ph = ['bathymetry']
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('atl24')
+      expect(req.atl24).toHaveProperty('compact', true)
+      expect(req.atl24).toHaveProperty('class_ph')
+    })
+
+    it('atl13x should include atl13-specific params', () => {
+      store.missionValue = 'ICESat-2'
+      store.iceSat2SelectedAPI = 'atl13x'
+      store.useAtl13RefId = true
+      store.atl13 = { refid: 42, name: 'test-lake', coord: null }
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('atl13')
+      expect(req.atl13).toHaveProperty('refid', 42)
+    })
+
+    it('atl06x should include atl06_fields when set', () => {
+      store.missionValue = 'ICESat-2'
+      store.iceSat2SelectedAPI = 'atl06x'
+      store.atl06_fields = ['h_li_sigma']
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('atl06_fields', ['h_li_sigma'])
+    })
+  })
+
+  describe('Universal params work for all endpoints', () => {
+    it('timeout and output should be included for X-series endpoints', () => {
+      store.missionValue = 'ICESat-2'
+      store.iceSat2SelectedAPI = 'atl06x'
+      store.useServerTimeout = true
+      store.serverTimeoutValue = 300
+      store.fileOutput = true
+      const req = store.getAtlReqParams(1)
+
+      expect(req).toHaveProperty('timeout', 300)
+      expect(req).toHaveProperty('output')
+    })
+  })
+
+  describe('photonProcessingAPIs set is correct', () => {
+    it('should contain all P-series and ATL03-based endpoints', () => {
+      const expected = [
+        'atl03x',
+        'atl03x-surface',
+        'atl03x-phoreal',
+        'atl03vp',
+        'atl06p',
+        'atl06sp',
+        'atl08p'
+      ]
+      for (const api of expected) {
+        expect(photonProcessingAPIs.has(api)).toBe(true)
+      }
+    })
+
+    it('should not contain X-series endpoints', () => {
+      const xSeries = ['atl06x', 'atl08x', 'atl24x', 'atl13x']
+      for (const api of xSeries) {
+        expect(photonProcessingAPIs.has(api)).toBe(false)
+      }
+    })
+  })
+})