Linked from
The 14 pages that link to Control volume, each with the reason it gives.
Mass balanceRelated: The balance depends on which region is treated as the system.
System boundaryRelated: Its enclosing surface makes system-boundary accounting explicit.
Mass flow rateRelated: Mass flow rate counts transport across the boundary of this analysis region.
Volumetric flow rateRelated: Flow rate is defined through the control volume's boundary surface.