The Booster Diminisher is a module of the Boosting locomotion; it provides a particular functionality for its parent Booster. If it is disabled or removed, then the relevant boosting functionality will behave as by default. Most modules can optionally affect change globally (for both hands' Boosters). Booster modules can also depend upon conditions specified by their dependencies settings.
The Diminisher module reduces the boosting force when raycasting farther away from terrain. The max distance and curve by which to diminish can both be set, and diminishing can be controlled per axis. Choose the recognized terrain type as defined by Terrain Response. You can also have boosting be prevented when raycasting is obstructed (when the Booster is inside of an unrecognized layer).
Diminishing is useful to keep the player from soaring away into the sky; instead, they can only fly so high off of the ground.
By default, diminishing only affects the y-axis. Furthermore, it is dependent on the player being affected by gravity, because diminishing feels like an effect of gravity to the player. You wouldn't want to have players restricted to a certain height above terrain when they are in the zero-gravity vacuum of space, as there is no terrain to raycast.