Two more PowerEdge servers running ESXi are used in this deployment: a witness host and a services host. A witness host is required for vSAN stretched clusters. It resides at a third site to act as a tiebreaker if a failure occurs at one of the other sites. The witness host can be a physical host, as used in this deployment guide, or an ESXi host running inside a VM. See vSAN Planning and Deployment Guide for VMware vSphere 7.0 Update 3 for more information.
The services host is used in this deployment for the initial vCenter server and DNS server locations, before they are migrated onto the vSAN cluster. The services host is also used for the OMNI VM after deployment.