RDM's are typically presented to the hosts where workloads would be run. This could be a single cluster or multiple clusters. VMware Clusters typically align with a Purity Host Group.
If you have multiple clusters, you'd typically want multiple Host Groups. If it is desired to present that RDM across multiple VMware Clusters, you'd assign the RDM to the multiple Host Groups in Purity (only those that would need access, instead of blanket access.)
VMware vVols can accomplish most (and more at some point) use cases that RDMs are chosen for today. Think of vVols as RDM's 2.0.
