This has been posted on the forum a lot already:
http://ftcforum.usfirst.org/showthre...me-Definitions
Read posts #5, #7, and #24 (pg. 3). These are the main ones that refer to goal possession other than the definition itself.
But, as you can see in the attached image in #24, you can constrain it on three sides. But the goal must have a way to separate from the robot (and vice versa) without excessive friction. The refs judge what is considered excessive friction.
However, in end game, you can possess the goal or lift, grasp, hold, etc (on 4 sides, if you want).
If you look at our robots on youtube, you'll see why we know the goal possession rules backward and forward.
...Well, it's been worth it so far. Hope this helps.