Key Mode - Hold
 

The other night I found something I have been looking for and now can't seem to find it ;)

You can see it in the attachment. It is Key Mode = Hold
Maybe it was only on the patch I was editing.

it's in the ARP edit menu.

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.