SeaDog, just an internal thing, I cant speak for every model and every radio but generaly imagine it as 3 seperate telephone books.
1 for say "Friends", 2 "Ausfish Members", 3 is for the "Fishing Fleet" that you belong too.
Example of groups,
You want to talk to a mate or a group of mates, just like text messaging you can select 1 person from that group or all persons in that group.
Usage, if you belonged to a fishing fleet and you ran aground on the GBR through negligence, illegal fishing etc, you should put out a DSC alert to "All Stations". The priority would normally be a "Mayday" DSC alert which is implicitly and automaticly addressed to all stations by default. (The vessel would be in grave and immenent danger) if they were to start transmitting using voice as it would not take the authorities long to pick up on it one way or another.
Now this alert would get the Master and the company in some serious Shite, big fines comming their way for parking on the reef!!!
What they can do, is select the group 3 that they require being "Fishing Fleet". Only the members of the group would get the message (Routine DSC, all though it's not routine), requesting assistance to get them off the reef without anyone else outside the group seeing or knowing that they have run in to bother.
Hope the info helped
See you on the 24th
Cheers Lloyd