084 Subfield $a - Classification number:

Subfield $a contains the classification number portion of the call number. Subfield $a is repeated to record an alternative class number. The alternative class number is recorded following the last subfield of any primary call number or classification. If the alternate class number also includes an item number, the item number is included in the same subfield $a as the alternate class number; it is not separately subfielded.

Examples:

084 ##$a014$2frbnpnav

084 ##$a016$a014$a018$a122$2frbnpnav