closed vs open system can mean two different things to me. i terms of a filtration, sumps are open systems. there is an open top to the sump which water can overflow if it gets too full and spill on the floor. in other words, its open to the air. a closed system refering to pumps/filtration would be like a canister filter
that has water go into and back out without any way for it to leak out. a closed loop is essentially the same thing, only typically only a pump and no filtration.
a skimmer must be used in an open system.
also, closed/open systems can refer to flow of nutrients. think of a tank and nutrients that are in it. the nutrients (like phosphate) have no way to leave once added. in this sense, tanks are considered closed systems becase there is no normal means of escape for the nutrients. some large aquaria are set up to use ocean water and a constant "water change" by continuously pumping in new and draining the old out to sea.