028 Subfield $b - Source:

Subfield $b contains the source of the publisher number. The source varies according to the type of publication and the type of publisher's number being recorded. For sound recordings and videorecordings, the label name is recorded in subfield $b. For printed music, the publisher's name is recorded in subfield $b.

Examples:

028 00$aM4X-31427$bColumbia

028 10$aJ-18961M-A$bCountry Line