The RevX has nothing to do with the FX position. The built-in HW DSP RevX is only for the monitor signal i.e. cannot be recorded (you have plugin RevX for track processing). Which mix is to be “hardwired” can be switched in the control panel.
Reverb can be rather demanding of processor power, so having this integrated in the DSP of the interface specifically for e.g. the artists monitor is very handy.