Gauge pressure unit: P [psig] = P[psi] - 14.6959
This meant to avoid confusion when the Pressure value is referenced in relative terms as a surplus to the standard atmospheric pressure, and in this case the psi unit is referred as psig: P [psig] = P[psi] - 14.6959
The incremental value is the same as psi: δP [psig] = δP[psi]
See also
Natural Science / Engineering / Measurement / Unit