This is an open mini ITX case. You can use it as is or as part of a custom case.
It was designed with M3 threaded inserts in mind for extra stability.
The larger holes are for inserts, the smaller ones for M3x16 countersunk screws.
Depending on your settings you might have to drill out the smaller holes a bit.
The holes in the back are for standard PSU screws.
The holes in the PCI-E plate are spaced for an extension cable I want to use. I don't know if other cables have the same spacing.