Gives the Arduino Uno a 0.5 inch stand off mount. Pockets are for through hole components clearance. Screws can thread right into the print.
Print with no support. Break off small support piece at the corner with the notch. Attach with 4mm or #4 screws from both ends.
The later versions of the Uno have 2 extra pins close to pin13, I had to turn down the screw head so that it would fit. I did this by threading the screw into an aluminum stand-off like in the picture, with a lock nut holding it in place so that the screw isn't threaded in completely. Chuck the stand-off into a drill and hold a file to the side of the screw head while running the drill until it is a small enough diameter to fit the Uno just right.