IGV: how is "Quality weight allele fraction" calculated?

Question

[omer.k]

I'm inspecting raw NGS reads from human using IGV. The allele frequency threshold is kept at the default 0.2.

Yet this is apparently "Quality weight allele fraction" (IGV Preferences) and not regular fraction (e.g. A/[A+C+G+T] ).

Where can I find the formula for this calculation?

Thanks.