Bar Absolute is based on a sealed unit, set to a nominal absolute vacuum. Zero in this case would be a complete vacuum, and at ‘ambient’ the reading on the display will be a true atmospheric pressure (usually between 970 and 1020 mb at sea level in the UK). A vacuum will read as e.g. 0.5bar (note this is positive not negative), and pressure values will read approximately 1 bar higher than bar gauge, as the baseline is atmospheric pressure rather than zero.
Bar Gauge is open to the atmosphere and uses atmospheric pressure as its base value – your pressure calibrator is therefore zeroed before testing. ‘Ambient’ will read 0, a vacuum will read as a negative (e.g -0.5bar), and pressure will read as a positive (e.g. 2.2bar). Recommended TQSoft settings would be a low of -0.5bar, high of 2.5bar and checkpoint of 2.2bar.