This is the E360 - Main Circuit Board Mounting bracket, it holds your circuit board off your enclosure, protecting it from shorts and is designed to maximize airflow.  Easy access to 4 mounting holes while the board is in place.  Each hole is for 5mm bolts. 

If you are using the recommended Power Supplies this is a must have, not only does it allow for easy mounting of the server grade power supply but it also holds the circuit board in place.  This bracket comes in 2 pieces front and rear.