D is for Direct Mount.
11-speed drivetrains were originally only found on high-end road bikes. In the past couple of years, mountain biking’s elite groups have adopted 11-speeds. This year Deore XT adopts the 11-speed drivetrain with its M8000 line of components. The Shimano Deore XT FD-M8000-D 2x11 front derailleur is only compatible with direct-mount 2-speed systems.
For its move to 11-speed compatibility, Shimano has made some important changes to its XT front derailleurs for the M8000 group. The new derailleur provides wider clearance for the new larger wheels and tires. It’s also compatible with the shorter chainstays found on many of today’s frames. Even with these changes, Shimano retains the XT front derailleur’s efficient cable routing to keep shift action light and quick.
This Deore XT Direct Mount Front Derailleur has a flat inboard surface with an upright rib that corresponds to the groove in the flat Direct Mount you'll find on your seat tube. One bolt holds it all together and your only adjustment will be up and down in relation to the chainrings. Since there's no way to twist the derailleur around the seat tube, it goes on lined up perfectly with the chainrings and your shifting should be spot on once the cable tension and limit screws are set. The limit screws are angled outward to facilitate adjustment for convoluted rear suspension designs as well.