code/_onclick/hud/rendering/plane_master_controller.dm

/atom/movable/plane_master_controller	Atom that manages and controls multiple planes. It's an atom so we can hook into add_filter etc. Multiple controllers can control one plane.
/atom/movable/plane_master_controller/non_master	Exists for convienience when referencing all non-master render plates. This is the whole game and the UI, but not the escape menu.