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"Plenum" is a fire code rating. It is sometimes identified by the
designation "CMP" meaning a communications plenum rated cable.
http://www.lockergnome.com/it/2005/04/01/what-is-plenum-cable/
Recommended for HVAC
Type cmp, catvp cl3p, fplp
P designates "plenum"
Plenum (OFNP - Optical Fiber Nonconductive Plenum)
Source: www.extron.com/company/article.aspx?id=decipheringapp
Here in Louisville, KY General purpose runs for about 75$ a box and Plenum is $250.