Certain bits within the slots perform error correction, making sure sure that what you send is what is received. Same way with data sent in frames on telephone land lines. The TDMA based IS-136 uses two slots out of a possible six. Now let's refer to specific time slots. Slots so designated are called channels, ones that do certain jobs.
Channels handle the call processing, the actual mechanics of a call.
Don't confuse these data channels with radio channels. One frequency to transmit and one to receive. In digital working, however, we call a channel a dedicated time slot within a data or bit stream. A channel sends particular messages. Things like pages, for when a mobile is called, or origination requests, when a mobile is first turned on and asks for service.