If you create a DRS affinity rule for your cluster, you can specify how vSphere HA applies that rule during a virtual machine failover.
The two types of rules for which you can specify vSphere HA failover behavior are the following:
VM anti-affinity rules force specified virtual machines to remain apart during failover actions.
VM-Host affinity rules place specified virtual machines on a particular host or a member of a defined group of hosts during failover actions.
When you edit a DRS affinity rule, select the checkbox or checkboxes that enforce the desired failover behavior for vSphere HA.
HA must respect VM anti-affinity rules during failover -- if VMs with this rule would be placed together, the failover is aborted.
HA should respect VM to Host affinity rules during failover --vSphere HA attempts to place VMs with this rule on the specified hosts if at all possible.
vSphere HA can restart a VM in a DRS-disabled cluster, overriding a VM-Host affinity rules mapping if the host failure happens soon (by default, within 5 minutes) after setting the rule.