Pavement Classification Number - it's a numerical measure of the load carrying capability of the surfaces. It looks at (among other things) the surface itself and the subsurface structure. It is compared with the Aircraft Number (ACN) which is a measure (in simple terms) of the load through each wheel. PCN needs to be greater than ACN ie surface needs to be stronger than the load.
A quick search via a well known search engine does not come up with a PCN - in some idle hours I'll see if I can come up with more info.