I think the "half seat" and "2-point" have several definitions depending on who you talk to! I've heard them used interchangably, have specific terms, etc.
The way I understand it and teach it (for the sake of clarity) is that 3 Point is 3 points of contact (leg, leg, full seat). Basically the position you'd be in if you were walking. 2 Point is 2 points of contact (leg, leg, butt in the air), basically the position you're in over the fence. The half seat (or medium seat or light seat) is somewhere inbetween, not quite so high out of the saddle or quite so forward out with my body. But for me (some people may disagree and that's ok) the half seat isn't an exact degree of angle with my upper body. (Just somewhere in between 2 and 3 point) Mainly b/c I change my body position a little through the course but still consider myself to be in a half seat. Going toward a jump or inbetween a line I'm light in my saddle, a little forward with my body, but on the backside of the fence if I need to steady or balance I'll stretch up, but not so much that I'm in a full seat. And I'd still consider that to be my half seat. This is how I ride a hunter anyways! When I ride a jumper I'd like for the horse to be a little bit more on his haunch, a little more collected, so I'll sit up a little more and ride more to the base of the fence. Generally though I'm usually still not in a true full seat when riding a jumper either, unless that horse is trying to take off with me... Then I'm doing anything in my power to keep control. :)
But like I said, everyone has their own interpretation and I'm not certain there's a true answer. This is just how I teach it!