The 9XX fields are available for local information, according to various descriptions by local systems, system vendors, MARC service providers, etc.  These fields are not described by MARC authority format.

Content designator history:

9XX - Equivalent Headings  [obsolete, 1997]  [CAN/MARC only]:  The 9XX heading fields were made obsolete in 1997 since the USMARC 7XX fields were already defined for equivalent headings.

See also:

Variable Data Fields