Shimano first started offering Top-Swing front derailleurs several years back as a way to offer the rider a lower profile and stiffer unit for a front changer. The name is derived from their function of the action and design of the derailleur - the cage swings above or on the top of the clamp, hence Top-Swing. Conversely, the classic style of derailleur would be considered Bottom-Swing. The Top-Swing design is the preferred style if will fit your frame, it will not always work with seat tubes that flair or are ovalized and the bottom of the tube by the bottom bracket juncture.
The XTR M985 is the top swing type with the lower seat tube clamp for the most solid shifting performance. The included shims allow it to fit a range of seat tube diameters and it works for either top or bottom pull cable routing. The M985 only works with a 2x10 drivetrain.