IP Multicast can work on Dense, or Sparse Mode. Dense, where all multicast traffic is flooded out of all the interfaces other than the one it came from (split horizon). It is easy to configure, and there are no planning with it. The down side is, it might create congestion in the network. Specially the WAN links, where every byte cost money. To tackle this issue, Sparse mode should be used.
Sparse mode, works by creating Shared-Tree that is rooted to the Rendezvous Point (RP). By default, all routers assume no one wants the multicast traffic, until they get a request for it.
To load balance the traffic, multiple RPs can be configured. But the question is, which RP will be responsible for which group of multicast streams?
The answer is simple, the RP will be chosen by Mapping Agent (MA) on the longest (most specific) Multicast group.
An example will clarify
R1(config)#ip access-list standard GROUP_1
R1(config)#deny 188.8.131.52 0.0.0.0
R1(config)#permit 184.108.40.206 220.127.116.11
R1(config)#permit 18.104.22.168 22.214.171.124
R1(config)#ip pim send-rp-announce loopback 1 scope 5 group-list GROUP_1
R2(config)#ip access-list standard GROUP_2
R2(config)#deny 126.96.36.199 0.0.0.0
R2(config)#permit 188.8.131.52 184.108.40.206
R2(config)#permit 220.127.116.11 18.104.22.168
R2(config)#ip pim send-rp-announce loopback 1 scope 5 group-list GROUP_2
What will happen, that R1 will be the RP for group 22.214.171.124 to 126.96.36.199. while R2 for 188.8.131.52 to 184.108.40.206. Group 220.127.116.11 will have no RP, so the only way for that group to work is to be configured in Dense mode.