What is the FILL feature

For myself, a new feature was revealed which is a basic feature but I haven’t noticed before. What exactly is the purpose of this “FILL”? At least it chooses the final chord itself?

Almost. In fact, even better :smiley:
It fills the rest of the pattern after the last entered chord. So if you have a progression of , for example, three chords, the Fill option will complete the five empty slots in the pattern with chords that match, more or less , the three original chords. Of course you can always arrange them manually after that.