and a threaded rod or some other form of adjustment for the tilt. I also find that having a second adjustment on the tilt plate to be useful - tilt at 90 degrees to the first - so that you can align dowels on the bell housing. Necessary when the gearbox weighs a ton or so.
It is the *critical spec*. Without it you can easily end up with a buckled clutch plate or difficulty getting the shafts aligned. The tractor I did recently had a dual clutch, one plate was for the PTO, so there were two splines to line up plus a pilot shaft. Flexibility is key.
If the jack isn't Ok for 100 jobs, it isn't Ok for 1.