Absolute air pressure is the measured atmospheric pressure, changes in weather conditions (the true measured air pressure of the current time and location). Absolute pressure is not corrected to sea-level conditions.(The air pressure decreases as you rise in altitude, the sea-level corrected pressure (the pressure your location would be at if located at sea-level) is higher than your measured pressure if you live above sea-level and lower than your measured pressure if you live below sea-level). Relative air pressure is the measured barometric pressure relative to your conditions.
The default of the REL pressure altitude is 1013 hPa,user can calibrate the relative pressure to the sea-level conditions.To determine the relative pressure for your location,locate an official reporting station near you(the internet is the best source for real time barometric conditions,such as weather.com or wunderground.com),and set your weather station to match the official reporting station.