A uniform system (open or closed) requires only a single state (a set of properties) to describe its thermodynamic state. The system state is an extended state that includes useful total (system) properties such as mass, volume, total energy, total entropy, etc., relevant to a system occupying a fixed volume at a given time.

When an evacuated propane tank is filled with propane from a large reservoir, propane within the tank can be described by system states (which, obviously changes during filling), while the flow at the neck of the bottle can be described by a flow state.

A uniform flow also requires only a single state to describe its thermodynamic state. The flow or surface state  is an extended state that includes useful flow (system) properties relevant to a flow such as flow area, mass flow rate, volume flow rate, and rate of transport of energy, entropy, etc.

In order to evaluate the mass flow rate of steam at a turbine inlet, the flow state daemon is more useful than the system state daemon.