[rcmolloy]

So after looking at the render that Vex Pro has posted, the snap ring groove seems to be retaining the sprockets running from the WCP/Vex Pro gearboxes to the shaft. The other side is tapped with a 1/4-20 in my best guess is primarily used to retain the wheels for the Versachassis set up that is shown by Vex.

Also to answer your other question, retaining clips are used to securely place some component on a shaft with precision. Special pliers are required to place them on shafts and most can be found from retailers such as McMaster, Grainger, etc. The main examples that you can see are in gearboxes and on miscellaneous shafts. If you would like a picture I can certainly snag one for you in regards to the gearbox example.

Hope this clears things up.