diff --git a/changelogs/application_service/newsfragments/1810.clarification b/changelogs/application_service/newsfragments/1810.clarification
new file mode 100644
index 000000000..66dab4954
--- /dev/null
+++ b/changelogs/application_service/newsfragments/1810.clarification
@@ -0,0 +1 @@
+Clarify that appservices should be notified of events relating to the sender_localpart user.
diff --git a/data/api/application-service/definitions/registration.yaml b/data/api/application-service/definitions/registration.yaml
index d9dfe7484..de5c1ba31 100644
--- a/data/api/application-service/definitions/registration.yaml
+++ b/data/api/application-service/definitions/registration.yaml
@@ -29,7 +29,9 @@ properties:
     description: A secret token that the homeserver will use authenticate requests to the application service.
   sender_localpart:
     type: string
-    description: The localpart of the user associated with the application service.
+    description: |-
+      The localpart of the user associated with the application service. Events will be sent to the AS if this user is the target of the event, or
+      is a joined member of the room where the event occurred.
   namespaces:
     type: object
     title: Namespaces