MultiFinder and System 7 and later supported faceless background applications similar to UNIX daemons, though using cooperative multitasking. Examples included Time Synchronizer (daylight saving time adjustment and remote time synchronisation), Software Update Scheduler, and Folder Actions (folder event handling). Faceless background applications were regular applications with the simple restriction that they did not show up on the application menu. As such, they were prohibited from opening a window: if they did so, the system would freeze. They were free to open global floating windows, however, since these could neither gain nor lose focus.
The only technical differences between a faceless background application and a regular application were that the "Only background" flag was set in the 'SIZE' resource.
The Control Strip in Mac OS 8 and 9 was an example of a faceless background application that displayed—in defiance of the term "faceless"—a global floating window to provide user interaction. The Application Switcher was another. However, the user was not aware at any time that the Control Strip was a running process; it was simply presented as an extra interface feature. The system simply described faceless background applications as system applications.
Language features in the Open Scripting Architecture (and thus AppleScript) were initially implemented as dynamically loadable plugins known as "scripting additions" or OSAXes. In Mac OS 8 and 9, these were augmented by faceless background applications that were loaded in the background on demand. Just as with regular applications, these applications were accessed using tell clauses: the global namespace was not updated as was the case with OSAXes. The operating system did not indicate the launch of such processes nor indicate whether or not they were running.
Read more about this topic: Extension (Mac OS)
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