How Do Car Radio Know the Call Letters?

I recently purchased a new vehicle, and one of the things I noticed was that on a few stations (about 5 or so), the digital display on the radio will show the call letters for the station. My question is, how does the radio know this? It must be part of the signal, but is it subsonic or supersonic in frequency and we just can’t hear it? Or is it somehow on a different simulcast frequency?

have a look here