I waffled between 3 and 5 and settled on 3 because I don't want addresses I never send mail to in the address book. (I use the To button when i address email much too often.)
I have some friends with addresses on every service and they don't use most of them for email, so why should I fill my...
DL = distribution list (hope I spelled it correctly).
GAL = global address list - this is the address list of all mailboxes on your exchange server
The admin needs to create the DL in the active directory on the Exchange server; its not a Contact group a user can create in their mailbox.