The Shimano XT M781 and M786 are designed to work with Shimano's 10 speed mountain groups. The M786 is for use with 2x10 while the M781 is for a 3x10 set up. The difference being in the cage profile. A 2x10 crank requires a smaller diameter cage with a different throw and conversely, a triple crank requires just the opposite.
These are traditional style derailleurs, meaning that the clamp is located above the cage. They are occasionally referred to as bottom swing whereas the cage swings below or on the bottom of the clamp, hence bottom swing. The clamp is black anodized and matches the rest of the 780 series components while the cage is coated for added durability and chain glide. Set screws are located on the outside of the derailleur for easy adjustment. When it comes to front derailleurs, no one does it better than Shimano.