I believe when the RB/FB crosses the plane of the goalline, it's a TD, because he has possession of the football, and is in bounds, before it crosses the plane
but a receiver generally doesn't have possession of the football before the ball crosses the plane of the goalline, since the last person who's able to establish possession is the QB, obviously, QB throws to WR, so therefore the wideout must establish complete control after he catches the pass, and have 2 feet inbounds inside the endzone...now if the receiver makes the grab before he reaches the endzone, with 2 feet in bounds, and has complete possession, all he has to do is stick the ball across the plane of the goalline, or touch the pylon, just like a RB, provided that his feet and body are inbounds, not touching the ground out of bounds on the sidelines, and it's a TD
hope that makes the rule less confusing...