562 Subfield $a - Identifying markings:

Subfield $a contains information concerning markings on the support or imbedded in the medium which can be used to identify the copy of the described materials (e.g., watermarks, annotations, or captions).

Example:

562 ##$aAnnotation in Wilson's hand: Copy one of two sent to John Phipps, 27 March 1897;$bCopy identified as Declaration of Dissolution, Phipps copy.