It's in the Common settings, it's usually set to Poly or Mono1-4, Hold is the last selectable option. Easily missed as most of us tend to just use Poly or Mono1 or 2 key modes.
It sustains any number of notes triggered at the same time until new note/s are triggered after releasing. It's an algorithmic hybrid of both Poly and Mono in a sustained vibe, easier to understand by trying it. A handy feature in certain circumstances.
PS > And another thing! Will the Ti|3 have user customisable/importable wavetables?
A ribbon-controller or XY-Pad might be nice, too, please! Thanks!