Fret files are used for putting the rounded shape back into frets after they are levelled. Fret files can also be used to grind down an individual fret if it's too high.
Nut files are used for cutting or adjusting the slots in the nuts of guitars and basses. Just choose the gauge that matches the string and cut the groove. If the string is bigger than the file, the file can be rocked from side to side to widen the slot slightly. #4050 set is more convenient for electric guitar strings and the #5001 is more convenient for acoustics. #5004 is designed for bass guitars. Fret end dressing file is used for deburring and smoothing off the ends of the frets on a guitar or other fretted instrument. This means the ends will be less sharp and less likely to be uncomfortable while playing the instrument.