Control volume; open system কী?

A control volume (or control surface) is a fixed region of space $\Sigma$ through which fluid flows; the fluid inside it at any instant is an open system, which exchanges mass with the outside through the inlets and outlets of $\Sigma$. The closed system to which Newton’s laws apply is the fluid that is inside $\Sigma$ at time $t$, followed to $t + \dd t$: it then occupies $\Sigma$ minus what has left plus what has entered.