The Shimano XTR FD-M985/986 Race Front Derailleur is designed to be used with XTR Race double cranks and their HG-X 10-speed asymmetrical chain. It's specific to the Race cranks because Shimano builds their front derailleurs to match their intended usage on specific chain ring sizes and total difference from top to bottom. The cage radius, cage length, and plate shapes are quite specific in order to improve performance. Let's suppose you installed this derailleur in conjunction with a triple crank. Aside from not providing optimal shifting, the chain would drag across the back of the cage when in the little ring and towards the small end of the cassette, and that would be supremely annoying.
The M985 is a low-clamp, top-swing style derailleur. It is useful for many full suspension bikes due to the lower mounting location on the seat tube. It offers relief for pivots and linkages in some applications. The M986 is a high-clamp, down-swing derailleur. It's the traditional design, and it works great on hardtails and many single-pivot full suspension designs with lower swingarm pivot locations.
Both styles benefit from a recent re-design from Shimano. The limit screws are now set into the linkage mechanism at an angle. This makes them much easier to access from the drive side of the bike, especially on complicated full suspension designs.
The Shimano XTR FD-M985/986 Race Front Derailleur is available in multiple configurations. The M985 is available with their Multi-Clamp system to fit 28.6, 31.8, or 34.9mm seat tubes, as an E-Type with plate for direct mounting to the bottom bracket shell, or as a low Direct-Mount to the frame or swingarm. The M986 is available as Multi-Clamp or as a high Direct-Mount. The Dual-Pull lever arm allows it to work with frames that use top tube or down tube cable routing.