Absolute Pressure（ABS):
Absolute barometric pressure is a comparison of how much pressure is exerted by the atmosphere compared to a vacuum, a space where there are no gases at all. The air pressure in a vacuum would be zero, since there are no gases to exert pressure on objects. Absolute barometric pressure is used primarily in scientific studies and industrial applications requiring precise data. Measurements known as "corrected barometric pressure" are used in most other applications, like weather reports.
Relative Pressure(REL):
Relative air pressure is known as corrected barometric pressure. It is a measurement of how much pressure an air column would exert at sea level. To determine corrected barometric pressure, an absolute air pressure measurement is taken, along with an altitude measurement. The relative air pressure of that column is the amount of air pressure it would exert at sea level, if it remained at a constant pressure all the way down. It is sometimes called relative pressure because it reports pressures relative to sea level.
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