Faceless Background Application or FBA - Carbon Dev for MacOS X Wiki

A Faceless Background Application or FBA is a Carbon application without a graphical user interface.

An FBA runs in the background while a user is logged in and it does not show up as a launched item on the Dock.

An FBA is usually a helper application, either launched by its main application or to be listed as an "Login Item" in System Preferences -> Accounts -> Login Items.

An FBA can use RunApplicationEventLoop() to enter the main event loop, with two caveats to remain in the background state:

Do not connect to the window server: That is any API that utilize user interface
Specify background property in the Info.plist as below:

<key>LSBackgroundOnly</key>
<string>1</string>

A closely related kind of application is one which assigns the value 1 to the key LSUIElement in the Info.plist file. Such an application does not appear in the Dock or the Force Quit window, and should not have a menu bar, but may present a user interface such as a global floating window.