What are application of a combination circuits?

A combination circuit is simply a circuit with multiple current pathways. There may be multiple signal pathways as well. Advantages might be that a single power supply can supply the operating voltage to a number of signal processing and mechanical drive circuits, as in a DVD player. The DVD must run the laser and pick up the reflected signal. It must then process the heck out of that signal to derive the video and audio information. And what opened and closed the drawer to accept the DVD? And what operates the sled on which the laser moves to track the disc? The military had a rough definition of a combination circuit. It might be one where two things must happen at once or must present "closed" pathways for another thing to work. The launching of an old ICBM out of the silo in which it was housed required two operators or launch control officers to turn their "launch" keys at almost the same instant to fire the missile. With those two examples, the idea behind a combination circuit should be easy to get a handle on.