Fault domains (FD) provide the core functionality of vSAN stretched cluster. The supported number of fault domains in a vSAN stretched cluster is three. The first Fault Domain can be referred to as Preferred data site. The second Fault Domain can be referred to as Secondary data site, and the third Fault Domain is the witness host site. Keep utilization per data site below 50% to ensure proper availability, if either the Preferred or Secondary site goes offline.