When you provision a virtual machine on an object-based datastore, such as Virtual SAN or Virtual Volumes, you must assign to the virtual machine an appropriate virtual machine storage policy that is compatible with the datastore. This assignment guarantees the optimum placement for virtual machine objects within the object-based storage. If you do not explicitly assign a 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 Virtual SAN.
The default storage policies for Virtual SAN and Virtual Volumes can be provided by VMware and user-defined. VMFS and NFS datastores do not have default policies.