Required DRS rules
The VM to host DRS rules can either be preferential or required. A required rule is stricty enforced and can never be violated, unlike a preferential rule. Required rules are often used to enforce host-based licensing. For example, if the software that is running in your virtual machines has licensing restrictions, you can use a required rule to run those VMs only on hosts that have the required licenses.
Here is an example (image source: VMware):
In the picture above you can see that we’ve created a DRS group for virtual machines named Group A and a DRS group for hosts named ISV-Licensed. The goal of this design is to force the VMs from Group A to run only on hosts in the ISV-Licensed DRS group because these hosts have the required licenses. But if the hosts in the ISV-Licensed group fail, VMs from Group A will not be moved to other host DRS groups.