PEP 825: clarify that value lists need to be sorted

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny

peps/pep-0825.rst

@@ -245,7 +245,8 @@ present in that wheel's filename.
 
 It has 3 levels. The first level keys are variant labels, the second
 level keys are namespaces, the third level are feature names, and the
-third level values are lists of feature values.
+third level values are sets of feature values, converted to lists,
+sorted lexically.
 
 
 Example
@@ -802,6 +803,10 @@ the variant metadata is mirrored in a JSON file published on the index.
 This enables installers to obtain variant property mapping without
 having to fetch individual wheels.
 
+Since JSON format does not feature a set type, sets are represented as
+sorted lists. Sorting ensures that tools can safely use equality
+comparison over dictionaries.
+
 The variant ordering algorithm has been proposed with the assumption
 that variant properties take precedence over Platform compatibility
 tags, as they are primarily used to express user preferences. This
@@ -988,6 +993,11 @@ and Zanie Blue.
 Change History
 ==============
 
+- TO BE PUBLISHED
+
+  - Clarified that feature values in ``variants`` dictionary are sets,
+    and that they ought to be sorted when serializing.
+
 - 17-Feb-2026
 
   - Initial version, split from :pep:`817` draft.