I believe that na is just a reference for wiring. as stated on apexis' website, and instructions for wiring their electronics up it shows the s14 ecu as being an n3-a wiring format for the pin out.
http://www.apexi-usa.com/pdfWiring/16.pdf
take note of the pin layout and how its set into the coding (N3-A, N4-A)
also this may help
http://240sxmotoring.com/srwiecupiand.html