diff --git a/changelogs/client_server/newsfragments/1909.clarification b/changelogs/client_server/newsfragments/1909.clarification
new file mode 100644
index 00000000..da6575cd
--- /dev/null
+++ b/changelogs/client_server/newsfragments/1909.clarification
@@ -0,0 +1 @@
+The `User identifier` object in `POST /_matrix/client/v3/login` contains additional properties that depend on the identification type.
diff --git a/data/api/client-server/definitions/user_identifier.yaml b/data/api/client-server/definitions/user_identifier.yaml
index 7e6eca9c..add848fd 100644
--- a/data/api/client-server/definitions/user_identifier.yaml
+++ b/data/api/client-server/definitions/user_identifier.yaml
@@ -18,7 +18,10 @@ type: object
 properties:
   type:
     type: string
-    description: The type of identification.  See [Identifier types](/client-server-api/#identifier-types) for supported values and additional property descriptions.
+    description: |-
+      The type of identification. See [Identifier types](/client-server-api/#identifier-types)
+      for supported values and additional property descriptions.
 required:
   - type
-additionalProperties: true
+additionalProperties:
+  description: Keys dependent on the identification type.