Flow Transmitter-mmH20

mmH2O is a convenient unit for flow measurement in operating plants, inH2O is also used, and related to liquid level measurement if you know the fluid density
It has to do with history. Early methods of Bernoulli based flow measurement involving a pressure drop across a orifice involved measuring the pressure drop with a U tube manometer. The manometer could be filled with mercury (Hg), water (H2O), even a liquid like alcohol or oil. Mercury is element with universally known properties and water, while not an element, also has universally known properties, so units in mercury and water are recognized standard units for pressure measurement. In more critical measurements the temperature of water is specified, because the density of water varies with temperature.

Whether the difference in manometer's column heights is measured in units of inches or millimeters depends on whether your national sport is football or soccer, but the difference in elevation between the two columns of a U tube manometer represent the differential pressure drop, upstream to downstream of the orifice plate.

U tube manometer used to measure DP across orifice.jpg