Pioneer SCSI-2 Reference Manual page 122

The ADR field shows the type of information encoded in the Q sub-Channel of this block as
defined in the CD standard.
MCV
If the MCV bit is one, the media catalog number field is valid.
There are identify numbers according to the UPC/EAN cording in the media catalog
number field. If all data in this field is ASCII 0, there is no media catalog number in the
field.
TCV
If the TCV bit is one, the ISRC field is valid. In this field, there are identify numbers
according to the ISRC standard. (DIN-31-621)
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Read Sub-Channel