PSIG stands for pounds per square inch gauge, which is a unit of pressure measured relative to atmospheric pressure. It is the difference between the pressure being measured and the atmospheric pressure at the location where the measurement is being taken. For example, if the atmospheric pressure at sea level is around 14.7 psi, and a tire is inflated to 30 psi, the tire pressure gauge will read 15.3 psig (30 psi - 14.7 psi) .

In contrast, PSIA stands for pounds per square inch absolute, which is a unit of pressure measured relative to a perfect vacuum. PSIA is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure.

In summary, PSIG is a unit of pressure measured relative to atmospheric pressure, while PSIA is a unit of pressure measured relative to a perfect vacuum.