Two layers are accessible to all apps: global and global_top (for popups and similar windows).
Three other layers are accessible to privileged clients (background, system and lock), for things that need to be on a different level than user apps, like the taskbar, system popups, menus and the lock screen.
Why are command-line utilities stored in "apps"?
And why are apps like "editor" or "terminal" top-level directories?
Command-line utilities now go in "utils".
GUI stuff now goes in "gui".
This includes: libui -> gui/libui, wind -> gui/wind, GUI apps -> gui/apps, editor&terminal -> gui/apps...
System services go in "system".
