The feature is called Autoset.

Available on (from memory):
     (UCX, UFX), UCX II, UFX II, UFX+
and the following preamps:
     Micstasy, XTC and 12Mic.

I use it often to find optimum level on UFX+ for Instr inputs (e.g. for e-guitar). But during use I turn it off to stay exactly on this level.
As my humbuckers are nearly at the same level I store the settings in a TM FX snapshot to have it ready for use.

You can leave it on for Mics, but if somebody touches the Mic hard, you might get a high peak input signal. Then Autoset automatically adjusts the input gain down. But then the rest of the recording might suffer from a too low input gain / level.

You need too check what makes more sense, to use it once to find proper gain with a good threshold (-6 dB if I remember right) and to keep it on or not.