These are some corner brackets to make quick fixtures out of 6mm MDF or plywood boards.
Glue is still needed to keep everything together, these are just for extra security and to make the ends pretty if you're terrible at woodworking like I am.
The number of 'branches' in the name specifies how many boards can go into the bracket.
The 'edge' brackets are meant to go on the bottom of the shelving unit to stay flush with the surface they're mounted on.
For the printing, place the flat side down on the printing bed.