The title speaks for itself. (I’m unsure about it.)
An engine is something comprised of many systems. For example, a Minecraft engine can contain a terrain generation system and an inventory system. I would call something an engine if it does a lot of the heavy-lifting for you, and can do a lot of stuff. A game engine, for example, handles all the complex rendering and physics stuff for you, so you can focus more on your game’s mechanics.
A system, on the other hand, tends to only do one type of thing. For example, a door system. It only handles and controls doors, nothing else.
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