A Possession changes holder (equivalently: 'owner') if the existing and new holder explicitly consent to it during the same Move; or if the new holder wins a duel against the existing holder.

The holder of a Rule is the thing that can move it on the Board (if it's on the Board) and that would have zero Size when sharing a Cell with it. The thing that holds a Skyway is the Chair of that Skyway. The thing that holds the Knife is the Knifebearer.