ACD should be clear, pressing a programmed key on digital sets, agents log in to their agent group, either by DN or Agent-ID, calls routed to that group are then distributed to among logged in agents.
(500/2500 type) are analog phones. The feature LILO allows analog phones to log in to agent groups, too.
Not only phones, many analog devices. I used modems and some fax as ACD agents...
I don't know what "unassigned" means in that sentence, perhaps that you can connect anything you want to an analog port ;-)
97
Don't know, where does it come from... a book, a list of features of your switch...??