External storage can be used with Cloud Foundation on VxRail (NFS/iSCSI/FC) to compliment primary vSAN storage for workload domain clusters. This is known as secondary storage.
External secondary storage is primarily used for:
A Cloud Foundation workload domain secondary storage use case involves manually mounting previously provisioned external NFS, iSCSI, or FC storage to a Cloud Foundation workload domain cluster that was created by SDDC Manager where vSAN is the primary storage. Lifecycle management for external storage is customer managed and validated.