A NAC is a Network Access Code, which is basically like DCS/DPL for digital conventional channels. With a NAC, you could have a number of different "talkgroups" on one frequency. So MHFD could have NAC 1 as "Dispatch", NAC 2 as "Fireground", NAC 3 as "Hazmat Operations", etc. I don't know if the PRO96 decodes NACs - I know the Uniden radios don't. If you have a discriminator-tapped scanner, you could run kNACk, a program that is bundled with Unitrunker and specifically sniffs out the NAC.