The terms "Front" and "Rear" are critical to understanding how paper is skewing during folding. These terms are defined relative to the positioning of the paper in the paper path as it feeds and exits.
"High" means the distance from the nip of the fold roller to the stopper is too far on one end of the fence.
"Low" means the distance from the nip of the fold roller to the stopper is too short.
Two examples are shown below.
Example 1: High (Stopper Too Far From The Nip)
The black arrow shows the direction of paper feed from right to left. When the skewed sheet is opened, the Front edge is longer than the Rear edge.
Example 2: FM2: Low (Stopper too Close to the Nip)
The black arrow shows the direction of paper feed from right to left. When the skewed sheet is opened, the Front edge is shorter than the Rear edge.