[dmurray]

In regular chess it's incidental. It only comes up in relation to the touch-move rule and similar rules. And even there it's given special treatment: if you touch your king, then your rook, you must castle on that side if it's legal, instead of making any other legal king move.

In this variant, whether castling is considered a king move or a rook move or both has an effect on which moves are legal, so it needs to be spelt out.