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 
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.
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