What is this retainer ring? Shimano RD-9000 which is a bit dinged up due to me setting the bike down...with my feet still clipped in to the pedals



What does this ring do, can/should I snap it back in to place, and how?  I have already tried a hex key down the barrel and it wouldn't budge.
This is termed the "B-axle Cap C-ring."  It's a split ring and can be removed by widening the split and prying out.  It retains the rest of the cap assembly.  Shimano doesn't specify this as a separate part number in their technical documents, but rather lumps it into the "B-axle Cap Unit".  You may have to purchase the whole assembly to acquire this ring.

On page 18 of this Dealers Manual, one can check out the directions for removal of this assembly.  In your case it may be possible to snap the ring back in if there no damage that has misshapen it or it's seat in the cap assembly.