[Preemptive vs cooperative] The internal vs external distinction refers to the source of initiative in control flow; in preemptive scheduling, the initiative is external because it comes from a system component that is external to the units of concurrency; in cooperative scheduling, the initiative is internal. The internal vs external distinction also maps to other pairs of concepts like synchronous vs asynchronous, pull vs push, direct vs inverted, opaque vs transparent and local vs remote or distributed.