greatly facilitates the creation of formant filter systems. check the help patch for more in-depth explanation/usage example. basically it is a preset object with 5 vowels (a e i o u) for 5 different singer ranges (0-4:alto, 5-9:bass, 10-14:countertenor, 15-19:soprano, 20-24:tenor). select two of those vowels (s1 and s2) and you can xfade between them via the +c control.
EDIT: added formantshift, pitch and c2 inputs. see help patch for explanation. basically you can shift all the formants via the formantshift control or shift formants dependent on input frequency, and c2 is a crossfade between the formantshift and the pitch input.