Formant is a control that preserves timbre when pitch shifting, or something like that, which you probably know, but yeah I have just crossed this section of the manual myself… thought I’d quickly find an answer but alas the modulations are described, omitting the description for formant modulation!
I’m guessing this allows you to modulate the range of formant tone shifting
I suppose you could loosely refer to pg. 112, 179, 181.