In libraries, art galleries, museums and archives, an accession number is a unique identifier assigned to, and achieving initial control of, each acquisition. Assignment of accession numbers typically occurs at the point of accessioning or cataloging. The term is something of a misnomer, because the form accession numbers take is often alpha-numeric. In libraries, art galleries, museums and archives, an accession number is a unique identifier assigned to, and achieving initial control of, each acquisition. Assignment of accession numbers typically occurs at the point of accessioning or cataloging. The term is something of a misnomer, because the form accession numbers take is often alpha-numeric.