The Diameter client could be common to multiple enterprises/departments with different domain names such as 1.abc.com, 2.abc.com, xyz.com. In the case where each of these domains have different AAA servers, the NAS client will need to route the request to the appropriate domain based on the NAI(in the format email@example.com) received from the user.
Therefore the client needs to maintain a peer table (as configured).
The realm table will be required to map the target domain to the AAA servers, to help the client entity route the request based on the received NAI. Most NAS clients support NAS server grouping for redundancy/loadbalancing.
Sample config would be like,
1.abc.com, 10.10.10.2 , Up
2.abc.com 10.10.10.3, Down
1.xyz.com 220.127.116.11 Up
NAS server group