ISO
ISO 12944
Corrosion Protection of Steel Structures by Protective Paint Systems
Classifies the corrosivity of an environment from C1 to CX and specifies paint systems, surface preparation and durability expectations against it. It is why a coating specification names an environment category rather than simply a number of coats.
This is an explanation, not the standard. PipingInspector is not affiliated with ISO and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.