AB = A Battery, BB = B Battery (-24v). I don't know why they are called A & B though, and I'm not completely sure, but I think the main difference is one is filtered and one isn't. There's also AG and BG (A&B Ground). And of course RC and RG (Ring). Oh, L & LG (Intercom Lamp). And a whole lot more, probably. This is just what I've learned while here. Also I had to bring over 10v and 18v to the 66 block, the 10v is to feed the intercom lamp input, and the 18v goes to the intercom as well, for signalling output.