A setlist (also track list) is a pre-determined order of songs which are played at a live music event. If you are performing on stage yourself, you will know how important the setlist is for the success of a concert.
For GEMA, too, the setlist fulfils an important role. After all, it is only on the basis of a setlist that we can allocate the collected remuneration for a concert to the creators and publishers involved. That is why we need the setlist 6 weeks after the event at the latest. Event organisers are responsible for submitting the setlist, but they can delegate that task to the musicians, too. At the end of the day, it is them that know best what songs they performed.