rephrase

PROTOCOL.md

@@ -477,7 +477,10 @@ The following is an example `remove` action.
 ```
 
 ### Add CDC File
-The `cdc` action is used to add a file containing only the data that was changed as part of the transaction. The file might be a [Change Data File](#change-data-files) or a [Data File](#data-file) that does not contain any copied rows. When change data readers encounter a `cdc` action in a particular Delta table version, they must read the changes made in that version exclusively using the `cdc` files. If a version has no `cdc` action, then the data in `add` and `remove` actions are read as inserted and deleted rows, respectively.
+
+The `cdc` action is used to add a [file](#change-data-files) containing only the data that was changed as part of the transaction. The `cdc` action is allowed to add a [Data File](#data-files) that is also added by an `add` action, when it does not contain any copied rows and the `_change_type` column is filled for all rows.
+
+When change data readers encounter a `cdc` action in a particular Delta table version, they must read the changes made in that version exclusively using the `cdc` files. If a version has no `cdc` action, then the data in `add` and `remove` actions are read as inserted and deleted rows, respectively.
 
 The schema of the `cdc` action is as follows: