vSphere Cluster Services (vCLS) ensures that if vCenter Server becomes unavailable, cluster services remain available to maintain the resources and health of the workloads that run in the clusters.
vCLS uses agent virtual machines to maintain the cluster services health. The vCLS agent virtual machines (vCLS VMs) are created when you add hosts to clusters. Up to three vCLS VMs are required to run in each vSphere cluster, distributed within a cluster. vCLS is also enabled on clusters which contain only one or two hosts. In these clusters, the number of vCLS VMs is one and two, respectively.
The vCLS agent virtual machines cannot be stored on a replicated datastore managed by Site Recovery Manager. You must have a non-replicated datastore with a minimum free capacity of 6 GB.