You assign an 18M to a single channel. The channel is used for firing purposes. For example, if your remote is on channel 01 and you fire cue #1, any module on channel 01 will fire cue #1.
To change the channel on the 18M, press and hold the CH+ or CH- momentarily. There is a "bump protection" designed to protect against accidental channel changes, so a momentary press is required.