584 Subfield $a - Accumulation:

Subfield $a contains the rate at which the described materials are accumulating expressed as a ratio of volume to time period.

Example:

584 ##$3General subject files$a45 cu. ft. average annual accumulation 1970-1979.$a5.4 cu. ft. average monthly accumulation, 1979-82.$aCurrent average monthly accumulation is 2 cu. ft.