[LAWS][FONT=fs_blakeregular]The ball is in touch if a player catches the ball and that player has a foot on the touchline or the ground beyond the touchline. If a player has one foot in the field of play and one foot in touch and holds the ball, the ball is in touch.
[/FONT][FONT=fs_blakeregular]If the ball crosses the touchline or touch-in-goal line, and is caught by a player who has both feet in the playing area, the ball is not in touch or touch-in-goal. Such a player may knock the ball into the playing area.[/FONT]
[FONT=fs_blakeregular][...][/FONT]
[FONT=fs_blakeregular]A player in touch may kick or knock the ball, but not hold it, provided it has not crossed the plane of the touchline. The plane of the touchline is the vertical space rising immediately above the touchline.[/FONT][/LAWS]
If the ball has not crossed the plane of touch
(i) and is caught by a player in touch, the ball is in touch (and was put there by the previous person to play it, not the catcher),
(ii) and is knocked into the FoP by a player in touch. The ball is not in touch. If the ball is knocked forward, that is a knock-on.
If the ball has crossed the plane of touch
(i) and is caught by a player with both feet in the FoP, the ball is not in touch.
(ii) and is knocked by a player with both feet in the FoP
....(a) and goes forward, that is a knock-on. If the ball lands in touch, the opponents have a choice of scrum or lineout.
....(b) and does not go forward, it is in touch it it then lands in touch, but not otherwise.
That is my summary of that bit of law as it stands at present.