Gage pressure modules have one pressure fitting & measure the pressure with respect to atmospheric pressure. Differential pressure modules have two pressure fittings & measure the difference between the applied pressure on the high fitting versus the low fitting. Each module is clearly labeled for range, overpressure & media compatibility. Compatible with all multifunction & pressure calibrators (including: Fluke 754, 752, 726, 725, 721, 719Pro, 719, 717 and legacy 741, 743, 744, 701 and 702).