Number of RCA output jacks on the rear of the receiver, used for connecting external amps. The most common configurations are 2 channels (one pair of R/L stereo outputs), used for connecting a 2-channel amp; 4 channels (two pairs of R/L stereo outputs), used for connecting a 4-channel amp, and 6 channels, with the third pair of outputs generally intended for use with a subwoofer amp.