Mixed materials:

The specifications for Mixed Materials is used primarily for archival and manuscript collections of a mixture of forms of material.  Material may be monographic or serial in nature.  The designator code MX is often used in written MARC documentation for this material type.

A record created using the MX specifications is currently (as of 1994) identified when Leader/06 (Type of record) contains code p (Mixed material).

The Mixed Materials definition of field 006/01-17 and field 008/18-34 is used when the above condition exists.  See 006 and 008:  Mixed Materials for codes and character position descriptions.

Note:  Prior to 1994, Mixed Materials (MX) were referred to as Archival and Manuscript Material (AM).