Function adjust_minor_vi_and_vii_by_quality 

pub fn adjust_minor_vi_and_vii_by_quality(
    key: &Key,
    reading: Minor67Default,
    quality: ImpliedQuality,
    accidental: i8,
) -> i8

The accidental a numeral on the sixth or seventh degree of a minor key ends up with, given how the key is read and the chord the figure asks for: music21’s adjustMinorVIandVIIByQuality.

A minor triad on the sixth or a diminished chord on the seventh is one only the raised degree gives, so under the Quality reading such a figure takes a sharp and a major one takes the natural degree. Flat and Sharp decide without looking at the chord, and Cautionary reads an accidental already written as a caution rather than as a further change: a sharp says nothing more, and a flat cancels the raise, so the two meet at the natural degree. In a major key the accidental stands as written.