What is plenum cable?
"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 made of Teflon rather than PVC (which you will find on CM or CMR rated cable) 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.
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.