Activate this option in order to split geometry vertex Normals based on edge continuity.
This option only applies to MotionBuilder workflows to ensure visual fidelity of surface continuity. It lets you transfer hard edge information to MotionBuilder. This is required because of a MotionBuilder limitation. Do not use this option if you do not require hard edges in MotionBuilder, since this operation duplicates vertex information and converts geometry. If your workflow is 3ds Max to MotionBuilder, for character animations, animate, then merge the animation back into your original 3ds Max scene.
To avoid all geometry/UV issues when you transfer hard edges to MotionBuilder, do the following:
Exporting with “Split per-vertex Normals” disabled produces this result in MotionBuilder:
Exporting with “Split per-vertex Normals” enabled produces this result in MotionBuilder: