AACR2 provides the following definitions:

Extent of item:

The first element of the physical description area. It gives the number and the specific material designation of the unites of the item being described and, in some cases, other indications of the extent (e.g., duration).

Specific material designation:

A term indicating the special class of material (usually the class of physical object) to which an item belongs (e.g., sound discs).

The "specific material designation" (SMD) refers to the specific type of item in hand:

For instance, if cataloging microfiche the SMD would be "microfiche," as opposed to the " General material designation," which would be "microform."

See also:

11.1. Extent of item