The Fuji Guys released a video guide to the new Fujifilm X-T2 AF-C custom mode settings that makes the new settings very clear. The 5 presets of the X-T2 AF-C custom menu vary the Tracking Sensitivity/Speed Tracking Sensitivity/Zone Area Switching to maximize performance.
Tracking Sensitivity – Delay from when the camera loses sight of the subject it is tracking, to when it tries to pick up a new subject. So if the camera is set to 0, the camera will adjust the moment something moves in front of the subject, but if you set the camera to 4 the camera will pause.
Speed Tracking Sensitivity – The way the camera predicts the position of the subject as it is moving, while it is being tracked. If it is set to 0, the camera predicts a constant rate of speed, but if you have it set to 2 or 1, the camera will take into account changes of speed when predicting where the subject will move too.
Zone Area Switching – Only applies when the camera is set to zone AF mode and allows you to prioritize which part of the zone is in focus. So you can set it to center/front (closest to the camera)/auto (first subject the camera locked onto).
The video is loaded with good examples of these settings applied and explanations of when to use each of 5 presets if you don’t want to set your own custom settings.