These are options that are designed to handle some common issues with models.
Apply body rotation to center
This will take the rotatoin from torso and hips and apply it to the center bone. Most models don’t use center bone other than position offsets but some have the waist area rigged to center bone. Therefore requires the center bone to be rotated otherwise the waist area will be twisted.
This adjusts twist of arm bones so that they are not twisted too much. This is useful when motion incorrectly twist arm bones exceeding normal range.
Some models have hands that are too big. This option allows you to scale the hands to the size you want.
BVH Thumb Motion
Some BVH motion have strange thumb motion. This option allows you to scale the thumb motion down or disable completely.