Like anything, it depends on the specifics we're trying to achieve. Bridging is used in order to get non-routable protocols to talk to each other over different/separate interfaces.
Your "bridge-group 1" will be defined someplace else as "bridge 1 protocol ieee" most likely. Or it could be IBM protocol. or if a switch, it could be vlan-bridge protocol. That defines the underlying algorithms. Without going into too much hideous detail, it's all about what you need to accomplish! And if you have no burning need (e.g. don't know what it is) don't go beyond the default!
I'm guessing you're wanting to change to bridging (between ethernet/VLAN1 interface and teh ATM VPC) in order to run PPPoE directly from the client PC instead of from the DSL configuration.