090-099 fields are reserved for a locally-assigned call number for a series when all or part of the series is classified as a collected set or with the main series (646, Series Classification Practice, subfield $a, code c or m).
For interchange purposes, the practices used in fields 09X should be known by parties involved in the interchange
Field 090 - Local Call Number [OBSOLETE, 1982]
Field 090 was made obsolete in all MARC specifications when the entire 09X block of numbers was reserved for local use. Prior to that change, the field was defined for series call numbers. In USMARC, both indicators were undefined. In CAN/MARC, the first indicator values were: 0 (Current call number) and 1 (Earlier call number).