https://en.wikipedia.org/wiki/Radio_Data_System
PTY (programme type)This coding of up to 31 pre-defined programme types (e.g., in Europe: PTY1 News, PTY6 Drama, PTY11 Rock music) allows users to find similar programming by genre. PTY31 seems to be reserved for emergency announcements in the event of natural disasters or other major calamities.
TA, TP (traffic announcement, traffic programme)The receiver can often be set to pay special attention to this flag and, for example, stop the tape/pause the CD or retune to receive a traffic bulletin. The TP flag is used to allow the user to find only those stations that regularly broadcast traffic bulletins whereas the TA flag is used to signal an actual traffic bulletin in progress, with radio units perhaps performing other actions such as stopping a cassette tape (so the radio can be heard) or raising the volume during the traffic bulletin.
AF (alternative frequencies)This allows a receiver to re-tune to a different frequency providing the same station when the first signal becomes too weak (e.g., when moving out of range). This is often used in car stereo systems.
REG (regional)This is mainly used in countries where national broadcasters run "region-specific" programming such as regional opt-outs on some of their transmitters. This functionality allows the user to "lock-down" the set to their current region or let the radio tune into other region-specific programming as they move into the other region.
CT (clock time)Can synchronize a clock in the receiver or the main clock in a car. Due to transmission vagaries, CT can only be accurate to within 100 ms of UTC.