<xs:documentation>An inlet or outlet port associated with this unit.
If there is an internal network then the name of this port must match the name of
an external port for that network.
Any properties (e.g., volume, pressure, temperature) that are assigned to this port
are inherently assigned to the corresponding external port on the internal network.
That is, the ports are logically the same port.
Similar to a node, there is no pressure drop across a port.
Also similar to a node, conservation of mass exists across the port and
the flow direction across the port can change over time if the relative pressures
across connected units change.</xs:documentation>