BYOND games used to have very limited interface options, all effectively sharing the same layout. In BYOND 4.0, skins were introduced, allowing developers more control over the layout.
A skin consists of macro sets for keyboard/gamepad input, menus, and windows and/or panes. All of these are considered controls that a game can interact with via winset(), winget(), output(), and a few other procs.
About the simplest possible skin is a single window with a single map control, and a single macro set.