update

aip/client-libraries/4235.md

@@ -38,19 +38,21 @@ If the aforementioned requirements are met for a given field, client library
 generators **must** enable automatic population of said field in the
 generated client.
 
-If the field value is not equal to default value at the time of sending the
-request, implying it was set by the user or if the field has explicit presence
-and is set by the user, the field **must not** be automatically populated by the
-client.
+The field **must not** be automatically populated by the client if:
+
+- The field does not support explicit presence, and its value is not equal to
+  the default
+- The field supports explicit presence, and presence indicates that it has been
+  set (even to the default value)
 
 If a field is specified in the `auto_populated_fields`, but does not meet the
 structural requirements, the client library generators **must not** enable
 automatic population for that field. Client library generators **may** emit an
 error during generation.
 
 Client libraries **must** reuse automatically populated values for retries of
-the same request. In other words, the automatically populated fields
-**must not** be regenerated for each RPC attempt with the same request message.
+the same request. In other words, the automatically populated fields **must not**
+be regenerated for each RPC attempt with the same request message.
 
 [AIP-155]: https://google.aip.dev/155