PMM stands for Pseudo Millimeters
Sketchup was created and remains CAD software for Architects. When used for working in small dimensions as I do for models there is a problem with drawing a radius below 0.018″.
I get around this lime by setting my units to Decimal Inches in Sketchup (Window >> Model Info >> Units). I uncheck “Display units format” and then model in Millimeters. If I draw a square that is actually 1″ x 1″ I do so as 25.4 x 25.4. Sketchup thinks I am making a square 25.4″ x 25.4″. When I export the STL file Sketchup does so in “Units” .. meaning it exports the STL square as 25.4 x 25.4 units .. simple numbers without any units set. When I import this into a 3d printing Slicer it expects millimeters .. and happily takes that STL file as such.
This means that I am drawing in CAD at 25.4 x the actual size in millimeters. That 1″ x 1″ square is being drawn 25.4″ x 25.4″. That 0.018″ radius is actually now 0.4572″. The actual limit therefore is 0.018/25.4 or 0.0007″ .. way smaller than I ever need.