SGS just means long cage derailleur. This model is the Official Derailleur of Trek 520 and Salsa Marrakesh Long Cage Shadow Derailleur.

Availability:
 - No longer available at MEC
 - Currently unavailable at Amazon
 - Evans says it is discontinued
 - Jenson says "no longer available"

Mount: Direct Mount (conventional), but then it says "Direct Mount Compatible". You can clearly see the B-Link in the photo. To mount on a conventional (standard) mount, you would leave the B-link attached. To mount on a "direct mount" frame, you would remove the B-Link and directly bolt the derailleur to the frame hanger.

This derailleur is referred to as a "shadow" derailleur. Shadow technology is already present on many of Shimano mid-level and high-end derailleurs. ... Shimano explains it to us in detail: it features a switch at pulley cage allowing you to enforce a heavier spring tension. The system works the same way as a clutch, facilitating a very accurate positioning of the cage.
 - the cage is inside the plane of the chainstay and dropouts.

With the RD Plus system activated, it is difficult even to move the pulley cage by hand. A small disadvantage comes when moving up the cassette, as shifting becomes a bit heavier, as the lever becomes slightly heavier to operate.