Mobile/Manufactured homes

Can someone please explain to me what this means:
gross floor area: all space, wall to wall, including recessed entries not to exceed 5 square feet and areas under built-in vanities and similar furniture. Where the ceiling height is less than that, the floor area under such ceilings should not be included. The floor area of closets should not be included in the gross floor area.

Where the ceiling height is less than that ?? For some reason I’m just not getting it. I hate to feel stupid but, I need help understanding what this is saying.
Thank you for helping

Yeah, you got my wheels turning also

Usually areas under 5’ are not livable space.

That sentence has been corrected to include a reference to the ceiling height specified in section 2.4.

http://education.nachi.org/show.php?element_id=3298&course_id=103

Thank you.