You can create replication domain as part of the cluster creation process, Ex. If you try to create a cluster using the WAS Admin Console this is the first screen that you get
If you Check the "Configure HTTP session memory-to-memory replication" check box during the cluster creation process, WAS will create a replication domain with default setting that has same name as that of the cluster
By default the replication domain will have be configured not to use encryption and will have single replica.
When you enable the "Configure HTTP session memory-to-memory replication", WAS will configure every Application Server in that cluster to use Memory to Memory replication for Distributed session, if you look at the settings of Memory to Memory replication you will see this
The replication domain is set to cluster1, the same domain which is created as a part of the cluster creation process and the Replication mode is set to Both client and server i.e. each server in the cluster will act as both client and server and copy each others HTTPSession data.