What is a Pneumatic Signal? why 3-15 Psig is using and standard value ?

A pneumatic signal is a pressure signal. many a times due to blockages or other reasons, pressure reaching the instrument may be zero where in fact it is not. Here is where the offset comes in. In the 3-15 psig range, no signal is indicated by a 3 psig pressure. A zero reading on the output will clearly indicate that some error has occurred somewhere. And regarding 15 psig limit, most instruments operate safely upto around 20 psig pressure. So incase it is over-pressurized, it will not damage the instrument uptil atleast 20 psig.

Pneumatic Signal is  pressure of a gas (or air) in a pipe, instead of electrical current .

It is difficult to control very low pressures accurately with a simple regulator, plus you have to provide vacuum to allow for calibration and measurement hysteresis & repeatability errors. thats way 3-15 psig used .