Heat and/or work can be directed into or out of the control volume. But, for convenience and as a standard convention, the net energy exchange is presented here, with the net heat exchange assumed to be into the system and the network assumed to be out of the system. If no mass crosses the boundary, but work and/or heat do, the system is referred to as a “closed” system. If mass, work, and heat do not cross the boundary (that is, the only energy exchanges taking place are within the system), the system is referred to as an isolated system. Isolated and closed systems are nothing more than specialized cases of the open system.