The current top-of-the-line 9 speed rear derailleur made by Shimano.
For wide range gearing—like for touring or trail riding.
Shadow is Shimano-talk for a rear derailer whose body doesn't stick out as far, and is therefore theoretically less likely to run into a rock. It was probably an overreaction to an incident like that, and in a rare circumstance it has to work, but for it to "work" you'd have to pass the rock with your body and legs, and then turn a corner just the wrong way and hit a rock.
Shadow rear derailers don't have barrel adjusters. That's a bigger deficit than the "shadow" part is a benefit, but as long as you have a RapidFire shifter with a barrel adjuster, you don't need another.
Shifts to 36t, or up to 45t with a derailer-xtender tab. An excellent choice, always. Nobody ever squawks about a Deore—the Toyota Camry of rear derailers.
Shimano code name RD M592 SGS
---------Feb 13, 2020 NOTE:
These 9 speed Shadow Deores will soon to be out of production, supposedly sometime this year, so stock up if you like it lots. The Shimano Deore replacement for this won't be 9s.