Abbreviation: FC = "Focus Clutch Mechanism" (Tokina)
The abbreviations means "Focus Clutch Mechanism"
Explanation from Tokina
Tokina AT-X PRO series lenses all feature the patented “Focus Clutch” Mechanism for switching the lens between auto focus and manual focus modes.
The manual focusing ring can move (be snapped) back and forth between an AF and MF position. When the focusing ring is forward in the AF position, it is not engaged to any of the internal focus gearing and will turn freely. Without the added weight of the metal ring the camera can auto focus the lens more quickly and smoothly.
For manual focus, simply rotate the focus ring all the way to one side or the other on the focus travel, either infinity or it’s closest focusing distance, then pull back (towards the mount plate) on the manual focus ring. While pulling back, rotate the ring from one side of the focus travel to the other. When the gears align, the focus ring will snap back into the MF position and the lens can be focused manually.
To return the manual focus ring to auto focus mode, simply snap the ring forward from any point.