Optional. Has different meanings, depending on the upload action:

Upload Action


Create folder

The ID of the folder created on the server.

Upload file,
Upload file first chunk,
Upload file middle chunk,
Upload file last chunk,
Delete File

The ID of the parent folder of the file that was uploaded or deleted.
In case the upload request contains more than one file, this header field
will return the ID of the parent folder of the last file of the set.