This field identifies the format of the COMMAND-AREA as a 20-byte or a 28-byte Release 4.x format, or a 72-byte Release 5.0 and above format.
TB-FORMAT = 0 for a 72-byte extended COMMAND-AREA
TB-FORMAT = A for a 20-byte or a 28-byte Release 4.x COMMAND-AREA.
Note:
A 20-byte command area is acceptable when TB-FORMAT = A as long as the application does not reference a VTS-TSR, in which the command area is either of a 72-byte (Release 5.x format) or a 28-byte (Release 4.x) format.