There are 16 audio output jacks on the NN-XT's back panel - eight separate stereo pairs. When you create a new NN-XT device, the first output pair (1L & 2R) is auto-routed to the first available channel on the audio mixer.
The other output pairs are never automatically routed. If you wish to use any of the other output pairs, you have to manually connect them to the desired device - typically a mixer channel. The basics on Routing is described in the chapter "Managing the Rack" in the Getting Started book.
Note that when you use any other output pair than the first, you also have to route one or more zones to it if you want it to actually output sound, since all zones by default are routed to outputs 1 & 2.