Hi, I have a SL16R III and a SL24 III. As far as I've seen, both units will run in stage box and monitor mixer mode.
In stage box mode you lose UC control over the unit that is acting as the stage box. It basically just becomes additional ins and outs. You lose access to the internal FX and GEQ's etc on the mix outputs. (Perhaps you slightly reduce you're overall round trip latency because of this?)
In Monitor Mixer mode you can still access the stage box unit via UC and you can still use that unit's internal FX and GEQ's.
Bear in mind that your 32 channel console still only has a maximum of 40 inputs. The channels towards the end of the mixer are not flexible in their input type. Yes, you can route network signals to them and they'll receive them OK but you can't control a stage box mic preamp from a FOH mixer's tape input (for example). The channel path is locked to behave as a stereo, line level channel source.
So in Monitor Mixer mode you can do some tricky routing and get your full 40 mic inputs (if that's what you're wanting to achieve) but you'll have to control a lot of those channels from UC exclusively and you'll struggle to deal with them on the FOH mixer end.
If you simply want to use your mixer in the pit to replace 16 of your 32 mic inputs then your FOH mixer will remotely control the 'stage-box' mixer's preamps and then source the signals from there over AVB.