Why is OP important in PCB production?

When having discussion with PCB maker, you must have had heard that PCB factory is following the specification and instruction in OP when producing the PCB of your design.

OP is the producing standard that engineer inputs all your requirements into. It is like an ID of your PCB design. Including the PCB size, layer count of PCB, copper thickness of inner layer and outer layer, surface finish type, solder mask color, silk screen color, V-cut or mouse-bite, quality level requirement, and other types of requirement that is related to your PCB design.

So is OP important to the PCB production? The answer is surely positive because all things related to PCB fabrication is based on this OP!