Gauge: Relative to atmosphere. For TPMS, water level, HVAC, compressors. Most common. Absolute: Built-in vacuum reference. For barometers, altimeters, GPS altitude, weather stations. Unaffected by atmospheric changes. Differential: Measures pressure difference between two ports. For air/gas flow, filter monitoring, respiratory devices (ventilators, CPAP, spirometers). Bidirectional Differential (50xx): Measures both positive and negative differential. For precision medical flow and HVAC. HBGK covers all types at multiple ranges.