852 $p - Piece designation:

Subfield $p contains an identification of a single piece when the holdings information does not contain an 863-865 (Enumeration and Chronology ) or 876-878 (Item Information) field that contains a subfield $p (Piece designation). The designation may be an identification number such as a barcode number or an accession number. Example:

The piece designation number may be preceded by an uppercase B or U to specify whether the piece is bound or unbound. When no piece designation exists, a double slash ( // ) may be recorded in subfield $p to signify that the field related to a piece.