• NSNormalWindowLevel - The default level for NSWindow objects.
• NSFloatingWindowLevel - Useful for floating palettes.
• NSSubmenuWindowLevel - Reserved for submenus. Synonymous with NSTornOffMenuWindowLevel, which is preferred.
• NSTornOffMenuWindowLevel - The level for a torn-off menu. Synonymous with NSSubmenuWindowLevel.
• NSModalPanelWindowLevel - The level for a modal panel.
• NSMainMenuWindowLevel - Reserved for the application�s main menu.
• NSStatusWindowLevel - The level for a status window.
• NSPopUpMenuWindowLevel - The level for a pop-up menu.
• NSScreenSaverWindowLevel - The level for a screen saver.
• kCGDesktopWindowLevel - Under the desktop. (Still accepts mousedown so desktop is useless where the window is located)

Another useful way to specify window level is with -[NSWindow addChildWindow:ordered:]. This allows you to place a window always directly over or under a related window (and will also cause drags of the parent to drag the child as well). Think of a sheet.

see also

*WindowAlwaysInFront *SplashWindow *GlobalFloatingWindow *PopOutWindowFromTitleBar