Nullable singleton should have identical semantics to a nullable single-value contained navigation property. If a singleton is null, a service may support upserting it via PATCH or PUT, and can announce  this with annotation Capabilities.UpdateRestrictions/Upsertable:true , see ODATA-1005 . If a singleton is nullable, a service may support deleting it via DELETE. By default nullable singletons are deletable, and the service can announce that it is not deletable with annotation Capabilities.DeleteRestrictions/Deletable:false , see ODATA-1005 . (Note: the default for this annotation is Deletable:true because it also applies to entity sets and has been around for some time.)