This lil' doodad goes on the non-drive side of a stock Sram GXP bottom bracket to cover up the bearing and properly space the left crank arm during installation.

One is included with the Sram GXP Team bottom bracket but people often lose them and we've seen many a set of used Omniums get passed around without one.

If you're missing one, you'll definitely know it because there will be side-to-side play on your Omnium or S300 cranks even when the bolt is tightened all the way. 

And yeah...we know your original one was black. The replacements are red. 

This piece is not required when using an aftermarket GXP bottom bracket (like Phill Wood or Chris King), only when using the stock BB.