When creating Virtual Machines to Hosts, the affinity or anti-affinity rules can be set to be required or preferential, as shown in the following screenshot of the dropdown box from the Create VM/Host Rule wizard:

The VM/Host affinity and anti-affinity rules are as follows:

• Must run on hosts in group: This is a required VM to Host affinity rule, and the virtual machines must run on the specified hosts. This rule will not be violated, even in an HA event.
• Should run on hosts in group: This is a preferential VM to Host affinity rule, and the virtual machines will run on the selected host when possible. The virtual machines can run on hosts outside of the group if necessary (for example, in an HA event).
• Must Not run on hosts in group: This is a required VM to Host anti-affinity rule, and the virtual machines will not run on hosts within the group. This rule will not be violated, even in an HA event.
• Should Not run on hosts in group: This is a preferential VM to Host anti-affinity rule, and the virtual machines can run on hosts within the group if necessary (for example, in an HA or maintenance event).

Once the VM/Host rules have been created, vSphere DRS will apply the rules when managing virtual machine placement and resource balancing within the vSphere cluster.