To use NukeX, you need both a Nuke and a NukeX license. The NukeX license won’t replace your Nuke license, which ensures you can still run previous versions of Nuke.
The procedure for obtaining and installing a NukeX license is the same as that for Nuke licenses. See Licensing Nuke on Windows.
NOTE: Two Nuke Assist licenses are included with NukeX maintenance licenses. See Nuke Assist on Windows.