The reference modes are Gauge,Absolute and Differential.
Referenced to atmospheric pressure.
0 means your test pressure is the same as atmosphere.
Gauge mode is the most common reference mode.
Can be both positive and negative.Usually the easiest to measure.
Referenced to a perfect vacuum.
0 means that it is a perfect vacuum.Absolutely no molecules.
Can only be positive numbers. You cant have negative molecules in a vessel.
More difficult measurement.
Referenced to another pressure. Eg: line pressure.
0 means that there is no difference in the two pressure.
Gauge mode can be thought of as a special case of differential mode.
Can be a difficult measurement to make.
Can be both positive or negative.