Picture.Save QualityDefault

What does QualityDefault (-1) mean?
Which value is used for the quality if you use QualityDefault?

Did you try to search on Google using:

Save As JPEG Quality Default

My testings:
92 % is the default for GIMP (when the loaded file was saved as JPEG from Xojo),
90 % when the image comes from a screen shot (cmd-ctrl-shift-4 + mouse selected area) on macOS.