One may occupy an opponent's rook. This is called hostile occupation. One's superpiece then controls the move of the opponent's rook, combining it with its own. A superpiece may leave the occupying position to move independently from a rook, but a rook may not move independently while a superpiece occupies. A capture may be of a superpiece only.
Capture is at the occupying position where the capturing piece becomes the new superpiece.
A capture of an occupied rook or a hostile occupation results in both the superpiece and the rook being removed from play. Capture of a rook under hostile occupation sacrifices one's own rook. In a hostile occupation, one could use the opponent's rook to give check to, or stop a castle move of the opponent's king. A superpiece may not leave a hostile occupation if the opponent's rook is then in a position of check to the king. To promote, a superpawn must leave the occupying position on the rook by a normal pawn move to the last rank.