Gage pressure, absolute pressure, differential pressure difference.

Absolute pressure is absolute pressure (engineering, physics appellation is absolute pressure).

Differential pressure is the pressure difference (or the pressure difference).

For example:

A pool depth of 20 meters, asked the bottom how much pressure or pressure? Studied physics knows, 20 meters deep water to produce 2 kg/cm2 pressure (or pressure). In fact this is a differential pressure, even if the pool has no bottom of the pool will still be a atmosphere pressure (or pressure), so the bottom by the absolute pressure is the pressure of about 3 kg/cm2 (or pressure).

Supplement:

Gage pressure, refers to the local atmospheric pressure as the starting point for calculation the pressure. When the measured system pressure is equal to the local atmospheric pressure, the pressure table pointer refers to zero. The gauge pressure is zero.

Gage pressure, i.e., absolute pressure - atmospheric pressure = table.

Gage pressure "in the vacuum industry especially: using ordinary vacuum gauge (relative pressure gauge) measured gas relative pressure value, expressed in negative, refers to the difference of the measured gas pressure and atmospheric pressure. Also called negative pressure.