The WM/MCDI attribute contains the music CD identifier. This is a binary dump of the table of contents from the CD that is used to uniquely identify the CD.
This attribute is compatible with the ID3 frame, MCDI. The ID3 specification for the MCDI frame requires that only one such frame exist per file and that a valid TRCK frame exist. The metadata editor does not perform any validation for these rules. Also, the size of WM/MCDI is not limited to 804 bytes, as is the MCDI ID3 frame.