300 Subfield $3 - Materials specified:

Subfield $3 contains information that identifies the part of the described materials to which the field applies. Subfield $3 is placed either immediately before or after the physical description data.

Examples:

300 ##$a1 reel of 1 (37 ft.) :$bsi., b&w ;$c35 mm.$3dupe neg.

300 ##$3diaries$a3$fv.

300 ##$3correspondence$a3$fboxes.