Metadata of a host pool is replicated within a geography for resiliency. If the region your host pool is in goes down, it fails over to its replica. During this failover period, Azure Virtual Desktop doesn't accept new user connections to the session host VMs in that host pool until the failover is complete. Any existing sessions on the session host VMs in that host pool remain connected and unaffected. To learn more about how service resilience is implemented for Azure Virtual Desktop, see Azure Virtual Desktop service architecture and resilience.