Embodiments of communications apparatuses, systems, and methods are provided in this disclosure. In an embodiment, an apparatus comprises: at least one processor, and a memory storing instructions for execution by the at least one processor to perform operations comprising: sending, to a network node, a first message comprising a first network slice identifier corresponding to a network slice in a first network slice set, wherein the network slice is accessed by a terminal device to a first access-network node, wherein the network node is a first core-network node or a second access-network node to be accessed by the terminal device, and wherein the first core-network node and the first access-network node are comprised in a first communications system; and receiving, from the network node, a first response message comprising a second network slice identifier corresponding to a network slice in a second network slice set.