There is no NC M/A TAC-2 channel. On the P25 talkgroup systems, there is something called FD TAC-2. However, if you're referring to a department's own tac 2 channel, it is probably what's labeled fireground on the RR database. For example, when I scanned Long Beach, the chief said "All units switch to tac 2". What he was referring to was Long Beach FG (labeled as such in the RR database).
To sum up, tac 2 is generally that department's fireground channel.