1) is there a mm option that can be selected?
--------- It works in inches or millimeters. It will use and indicate whichever you have set in the job properties when you created it.
2) what does the entry for "perpendicular fret" do? (the default is "8", what does that mean)
---------- "8" means eighth fret. This is only relevant for fan/multi-scale layout. It determines which fret (or point on the fret board as it does not need to be an integer) the scales fan/diverge. That is it sets the fret that is perpendicular to the center line or parallel to the nominal nut. Some call it the parallel fret but on a multi-scale fret board nothing is parallel. The frets fan from the perpendicular fret.
3) Does "Blind frets" mean the slot stops short of the edge of the board by the specified amount?
4) Does "enforce width" enforce the string width plus specified side allowance or the board width parameters?
---------- The latter. It forces the board width to whatever is specified for the nut and 12th fret. That is, the gadget either calculates the fret board edges based on the string width -- or uses the specified board width.
In the latter case, enforce width, the string width is calculated based on the specified board width and the string Edge to String values. The string width is only relevant for the multi-scale calculations. It makes no difference for a standard/single scale length.