A control plane (CP) function for mobility management may receive, from a user equipment (UE), a message which indicates a registration request for registration to a network slice. The CP function may select, as an allowed slice ID, a first slice ID of a first network slice in which to register the UE. The first slice ID has a slice/service type (SST) value indicating an SST and a first slice differentiator (SD) value associated with a first level of service to be provided. The CP function may alternatively select, as the allowed slice ID, a second slice ID of a second network slice in which to register the UE based on unavailability of the slice type associated with the first level of service. The second slice ID has the SST value indicating the slice type and a second SD value associated with a second level of service to be provided.