I would assume that it is Smooth Interior Corrugated Polyethylene Pipe which is essentially what N-12 is. It is usually abbreviated as SCIPP, so this could be a typo.
Your specs should provide you with information regarding the pipe material to verify if N-12 is suitable.
If you haven't already, you should discuss your project with city/county and regional WI-DNR office. I completed a project in the La Crosse area late last year and found the DNR to be quite helpful. Working with them early in the project clarified my questions and any concerns they had with...