The VMware monitoring template Template Virt VMware has cluster and datacenter based group prototypes:

• {#CLUSTER.NAME} and {#DATACENTER.NAME} for hypervisors
• {#CLUSTER.NAME} (vm) and {#DATACENTER.NAME} (vm) for virtual machines

The group prototype names for virtual machines have (vm) added to them because groups discovered by different discovery rules cannot have the same name. However with nested host groups it would be more natural to change virtual machine group prototypes to {#CLUSTER.NAME}/vm and {#DATACENTER.NAME}/vm