code/modules/parallax/parallax_render_sources/parallax_render_source_parent.dm

/atom/movable/screen/parallax_render_source	Parallax render sources are screens that hold the appearance data for a specific type of parallax layer; this appearance is then drawn to client parallax layers using render targets and sources. This permits the appearance of a specific parallax layer to be edited at runtime and efficiently distributed across all applicable clients.