As far as I know, TA stands for Traffic Announcement, indeed RDS based. E.g. in Germany and Holland this is frequently used. Example - If I'm listing to a CD and the radio receives a TA signal, it switches to radio mode (usually the hourly news update has finished and the traffic jams etc are listed) and stays tuned until the TA off signal is received.

TMC is a complete different setup - navigation equipment frequently uses TMC (either as addon or build-in). On my TomTom Go (with separate TMC receiver cable) displays actual route incident information (e.g. accident on highway A2, anticipated delay 5 minutes).

Wikipedia has a page on TMC

http://en.wikipedia.org/wiki/Traffic_Message_Channel

Jelle