When you provision a virtual machine on an object-based datastore, you must assign to the virtual machine a compatible virtual machine storage policy. The object-based datastores include vSAN or Virtual Volumes. This assignment guarantees the optimum placement for the virtual machine objects within the object-based storage. If you do not explicitly assign the storage policy to the virtual machine, the system uses a default storage policy that is associated with the datastore. The default policy is also used when the policy that you assign does not include rules specific to Virtual Volumes or vSAN.
VMware providers the default storage policies for vSAN and Virtual Volumes. Alternatively, you can define the default policy. VMFS and NFS datastores do not have default policies.