020 Subfield $z - Canceled/Invalid ISBN:

Subfield $z contains a canceled or invalid ISBN and any parenthetical qualifying information. Each canceled/invalid ISBN is contained in a separate subfield $z. If no valid ISBN exists, subfield $z may be used alone in the record. ISBN (invalid) and the embedded hyphens may be generated.

Example:

020 ##$z0835200028 :$c$10.00

020 ##$a0835200019 (pbk.) :$c$2.95

[Two ISBNs associated with one catalog record, one of which is invalid.]