Historical Rules · 17 June 2011
Rule 714.5a
Official source text from the 17 June 2011 Rules release
At the end of a subgame of a Planar Magic game, the face-up plane card is turned face down and put on the bottom of its owner's planar deck. Then each player moves his or her planar deck from the subgame command zone to the main-game command zone and shuffles it.