Yes, the Busses refer to Aux outputs. Though all the routing on the XR18 is flexible through the software patch.
This is one of the areas where the flexibility of digital mixers can catch out people moving from the less flexible analog world.
The XR18 has mix busses 1-6, which by default are routed to the physical outputs aux 1-6 - which would make your mix 1-6 = aux 1-6 assumption correct.
However, the routing options in the mixer allow all sorts of alternative routing to be set up, and different purposes for mix busses. Yes, most live uses will use the aux outputs for monitors using the mix busses. But, in the routing options, you can do things like routing an input channel direct to an aux output. Or you can configure one or more mix busses to be subgroups, which you may or may not want to come out of the corresponding aux output.
Think about it hard, and look at some of the videos on YouTube, and it'll start to make sense.
But then remember that, with the default setting the mix busses 1-6 on the XR18 will almost always be used as monitor sends to aux 1-6
