Merge pull request mendix#10450 from joostverhoog/external-action-list-of-primitives

List of primitives in external actions with open type parameters

Author: Yiyun333

content/en/docs/refguide/modeling/application-logic/microflows-and-nanoflows/activities/integration-activities/call-external-action.md

@@ -81,6 +81,37 @@ If you have selected a variable for an entity that is an open type (`OpenType="t
 The ability to add additional attributes was introduced in [Studio Pro 11.5.0](/releasenotes/studio-pro/11.5/).
 {{% /alert %}}
 
+#### Add/Edit Additional Attribute
+
+An additional attribute can have one of the following types:
+
+* Boolean
+* Date and time
+* Decimal
+* Enumeration
+* Integer/Long
+* String
+* List of Boolean
+* List of Date and time
+* List of Decimal
+* List of Enumeration
+* List of Integer/Long
+* List of String
+
+For types **Enumeration** and **List of Enumeration**, Studio Pro lets you choose the enumeration. This must be an external enumeration of the action's service.
+
+For types that are not a list, you type an expression of the given type.
+
+For list types, you:
+
+1. Choose an entity.
+2. Type an expression that is a list of that entity.
+3. Choose an attribute of that entity with the given type.
+
+{{% alert color="info" %}}
+List-types were introduced in [Studio Pro 11.6.0](/releasenotes/studio-pro/11.6/).
+{{% /alert %}}
+
 ### Associations to Include
 
 If you have selected a variable for an entity that has associations, you can select the associations you want to include in the arguments that you call the external action with.