AD RMS clusters are defined as a single server running AD RMS or a group of servers that share AD RMS publishing and licensing requests from AD RMS clients. When the first AD RMS server is provisioned in an Active Directory forest, it becomes a AD RMS cluster. More servers can be provisioned and added to an AD RMS cluster at any time.

There are two types of clusters: root clusters and licensing-only clusters. The first server in an AD RMS installation always becomes the root cluster. The root cluster handles all certification and licensing requests for the Active Directory Domain Services (AD DS) domain in which it was installed. For complex environments, licensing-only clusters can be created in addition to the root cluster. However, it is recommended to use only a root cluster and join more AD RMS servers to this cluster because root and licensing-only clusters cannot be used in the same load-balancing pool.

Additional reference