This is hanging support for Ikea KOMPLEMENT baskets (and probably some others) It can be used if you want to fix this basket from top, not from sides like it "should be".
Just see the picture.
Mounting holes for basket rails are 40mm from top (if you want to do it as on the picture - just from one side).
I've created a front "hook" (just left one) because basket rail was protruding (10mm) a bit from my closet, but for normal usage standard hook is enough, which can be used as both left and right support.
Hooks (for strength reasons) should be printed as are in STL files. Rail screws should be screw directly to the hook. For that reason, before installation, you should heat up screw a bit (50-60C) and create a thread with it (when not heat up - it may split the print).