Absolute Pressure Switch: a switch designed to actuate at points referenced to absolute zero.

Altitude Switch, an absolute pressure switch with actuation points referenced as a distance of altitude.

Gauge Pressure Switch is a switch designed to actuate at points referenced to atmospheric pressure.

Differential Pressure Switch is a switch that detects variations in a fixed difference between two pressures.

Vacuum Switch is a switch designed to actuate at a pressure below ambient. (pressure of the surrounding atmosphere).

Actuation Value is the difference in value between the switch point on increasing pressure and decreasing pressure.

Proof Pressure is the maximum pressure that can be applied without damage or calibration shift.