The estimate of the size of the audience relative to the total number of homes or people that can receive this channel. Coverage Area Ratings are used for each individual cable network. The Coverage Area Rating for one cable network cannot be compared to another cable network’s coverage area rating or a broadcast network rating. Only total U.S. Ratings or audience projections (estimated number of households or persons) can be compared between/among networks.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf