ok. let me re-phrase my question.

If one want to listen to Motorola trunked frequencies, what should
he/she understand about the difference between primary control
channels, alternate control channels and the rest of the channels
(i.e., "black" channels on radioreference.com)? Why should he/she
enter some of them in his/her banks but others not? What do these
channels do for trunking? What is their function/use?

(please keep in mind that I am *very* obtuse and even the obvious (to
others) needs to be explained (to me)).