External storage and Cloud Foundation have two important use cases: principal storage and supplemental storage.
By default, VCF uses vSAN storage as the principal storage for a cluster. The option to use NFS and FC-connected external storage is also available. This option enables administrators to create a workload domain cluster whose principal storage can be an NFS datastore or an FC-based VMFS datastore instead of vSAN. SDDC Manager provisions a workload domain (WLD) cluster by attaching a previously created NFS file system or FC LUN to it instead of using and configuring a vSAN storage for that cluster. In both cases, the administrator manually creates the external storage ahead of time. The network (IP or FC) must also be configured or zoned ahead of time so that the nodes that will be used for the workload cluster have access to the network. The mounting and formatting of the storage is automated as part of the SDDC Manager ‘create WLD’ cluster workflow. Importantly, Cloud Foundation does not provide extensible LCM capabilities for the external storage systems. That is still a separately managed process.
At the time of writing, Cloud Foundation on VxRail supports supplemental storage use cases only.