Open Loop/Closed Loop

[Johnny Reefer]

Just need a little help understanding the definitions of "open loop" and "closed loop".

I've seen, a few times now, the words "closed loop" and "sump" used in the same sentence. [Internal overflow(s)].

Up to now I always thought that any sump system is an open loop and that something like a canister filter, say, is a closed loop.

I'm pretty sure that I understand that a sump system with external overflow siphon boxes is an open loop, but I am wondering....
Is a sump system with an internal overflow considered a closed loop?

Thanx much,