public.openTypeCategories info should be complementary by default

[madig]

I think once you declare the key, there's no GDEF guessing done anywhere. It would be nice if you could declare categories just for a couple of glyphs and the guessing does the rest. If you want to exclude a glyph, you can set the category to "unassigned".

[anthrotype]

since it's a "public." key the discussion should happen at the UFO spec level, though it might be too late to change its semantics.

[anthrotype]

actually the spec currently leaves it to authoring tool, so I suppose we could implement what you suggested:

Note: If a glyph’s category is not defined in the dictionary, authoring tools may assign it to any glyph class in the OpenType GDEF Glyph Class Definition Table.