It’s been pointed out to me that the internal weight values of my lightest instances are too large and not distinct enough from each other. However, I can’t amend that in Glyphs, since the internal weights are picked from a menu with only very few choices (and often overlapping) choices. Could you please add a «Custom» field there to allow for arbitrary values, or at least expand the menu to a finer grid? It would be great to have 200 and 100 as available choices.
That custom parameter is called weightClass.
Good to know that there’s a workaround, thanks!
It would still be preferable if the menu worked as intended, though. 
Not as default. This is explained in chapter 7.3.3 on p.89 in the handbook.