A: In VMware vSphere's traditional DRS, affinity rules provide a way to define whether virtual machines (VMs) should, or shouldn't, be collocated on the same host as the result of load balancing. These rules ensure, for example, that your two domain controllers (DCs) never end up on the same host, or that two servers requiring high network connectivity always end up together.
Unlike traditional DRS, Storage DRS concerns itself with VM disk files. Affinity rules in Storage DRS can be used to ensure a VM's disk files are always migrated with each other (or are always on different volumes). They can also ensure that the disk files of two separate VMs end up together or segregated as a result of load balancing. If your vSphere environment runs VMs with any of these use cases, you'll want to configure one or more affinity rules to ensure their disk files always (or never) stay together.