088 Subfield $z - Canceled/invalid report number:
Subfield $z contains a canceled or invalid report number. Each canceled or invalid report number is contained in a separate subfield $z. If no valid report number exists, only subfield $z should be recorded.
Examples:
088 ##$aNASA-RP-1124-REV-3$zNASA-RP-1124-REV-2
[One valid report number has been canceled in favor of another.]
088 ##$zNASA-TN-D-8008
[Report number is invalid due to an error in assignment or printing on the item.]