“Plenum” is a fire code rating. It is sometimes identified by the
designation “CMP” meaning a communications plenum rated cable.

Plenum cable has a jacket usually made of Teflon rather than PVC and is used
in spaces designated for air-handling, such as drop ceilings that conceal
return air vents in office buildings. The Teflon gives off much less
poisonous gas than PVC when it burns. Thus by using plenum cable in
air-handling spaces one prevents poisonous gas from being created and spread
throughout the building through the air ducts during a fire. (That would be
really bad.)

Many commercial installations require the use of plenum cable. To make
certain you are installing cable that is up to code, you have to check with
your local fire marshal or building inspector. The laws vary from state to
state and town to town.

If you have a copy of the NEC (National Electrical Code) book, take a look
at section 800.51 (A). This states that one test used to define CMP cable is
the Standard Method of Test for Flame Travel and Smoke of Wire and Cables
for Use in Air-Handling Spaces. You can also take a look at the UL July 2004 Cable Marking
Guide (page 21 under CMP) to see the UL requirements for a CMP rating
based on this test.