One of seven types of items for which the MARC 21 format for bibliographic description establishes content designators for data elements. The category includesprinted, manuscript, and microform music, as well as musical and nonmusical sound recordings. Music materials may be monographic or serial. The code MU is often used in written MARC documentation to designate this material type.
– from Online Dictionary for Library and Information Science (ODLIS) by Joan M. Reitz http://www.abc-clio.com/ODLIS/odlis_m.aspx#music