Absolute pressure switch: a switch designed to activate at measuring points referenced to absolute zero.

Altitude switch: an absolute pressure switch with activation points referenced to a distance in altitude.

Manometer pressure switch: a switch designed to activate at points referenced to atmospheric pressure.

A differential pressure switch is a switch that senses variations in a fixed difference between two pressures.

A vacuum switch is a switch designed to activate at a pressure below ambient pressure (the pressure in the surrounding atmosphere).

Activation value is the difference in value between the breakpoint at increasing pressure and decreasing pressure.

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