UFO subsetting flag documentation

Would it be possible to document the glyphsLib subset / export flag key/value strings (or other types if used) that are used in your subsetter and where in the UFO source this lives so that these can be implemented directly in UFO source? It looks like it is maybe an array of glyph paths/names in a property list under a `com.schriftgestaltung.XXX` key within the lib.plist file of UFO?

https://github.com/googlei18n/fontmake/blob/master/Lib/fontmake/font_project.py#L322

Thanks!