Assuming your phone isn't controlled by VZW's PRL (preferred roaming list) this is what the following settings mean:
Home Only - means the phone will only work in the home area and won't roam
Automatic A - means that the phone will look for a signal on the "A" side of the cellular networks
Automatic B - means the same above, but the phone will look for signals on the "B" side.
Someone please correct me if I'm mistaken. :-)