i don't know if this answers your question but, the difference is Relative
Pressure takes into account your current location (i.e. Altitude), and
Absolute Pressure is pressure at sea-level. As for setting that parameter,
i think it differs from weather station to weather station. My weather
station, i simply had to enter the current relative pressure once, and now
its set.

Hope this helps

Mike

www.dudleyweather.pwp.blueyonder.co.uk