You've certainly noticed when look at individual BGP routes on your Cisco routers that the route is advertised to specific update-groups. It isn't obvious from that output just what those update-groups are.
Router#show ip bgp 126.96.36.199 BGP routing table entry for 188.8.131.52/24, version 50004440 Paths: (3 available, best #3, table Default-IP-Routing-Table) Advertised to update-groups: 2
BGP Dynamic Update Peer-Groups is a feature of IOS that groups together neighbors that receive the same updates. These groups are created automatically and do not require intervention from the user.
In order to see who is a member of an update-group, use the show ip bgp update-group command.
Router#show ip bgp update-group 2 BGP version 4 update-group 2, internal, Address Family: IPv4 Unicast BGP Update version : 50945923/0, messages 0 NEXT_HOP is always this router Community attribute sent to this neighbor Update messages formatted 15921628, replicated 15921628 Number of NLRIs in the update sent: max 1065, min 0 Minimum time between advertisement runs is 0 seconds Has 2 members (* indicates the members currently being sent updates): x.x.x.x x.x.x.x