Pressure sensor port as vacuum means?
The sensor is a differential pressure sensor. This means that it reports the difference of the pressures between the  two ports: P = P1-P2. The P2 pressue must always be lower than the P1 pressure because the device utilizes a single ended, positive supply so the resulting output of the P1-P2 calculation must always be positive. 

Viewed this perspective, P2 is a relative vacuum (lower pressure) to P1. If you leave P2 open to atmospheric pressure and the pressure on P1 is greater than atmospheric, the sensor will report guage pressure - the same pressure reported by an electromechanical, single ended pressure gauge such as a tire or water pressure gauge.

Another application for such a sensor is to monitor the condition of a forced air filter. If P1 monitors the "inlet" side of the filter and P2 monitors the "outlet" side of the filter, then a rise in P (P1-P2 )  indicates that the filter is becoming plugged. If P falls toward 0, this indicates a probable failure of the fan or the removal of the filter.