By default, BYOND displays all maps in top-down format, so world.map_format is set to TOPDOWN_MAP unless you say otherwise. This view means players are looking down on the map, and “north” corresponds to the top of their screen. (This can be changed by setting client.dir.)
A related map_format, used by older games, is TILED_ICON_MAP. This is also topdown but it handles icons differently.
In this form, the layer var behaves exactly as you would expect: Icons with a lower layer are drawn beneath icons with a higher layer. The only exception is when you use big icons, which will be drawn above any other icons on the same layer. Also an atom’s underlays will be drawn behind it unless their layer is changed, and its overlays will draw in front of it unless otherwise stated.
Topdown mode also guarantees that world.view or client.view will set the exact screen size used by the HUD, except for HUD objects that appear outside of the normal bounds.
Screen objects (also called the HUD) cannot be intermixed with topdown icons. They will appear on top of other icons, unless using a lower plane or a special layer like BACKGROUND_LAYER.