"Heavy Clothing" refers to FBI standards established in 1989: (1) layer of Denim (14.4 ounces/yard), (1) layer of Polartec 200 fleece, (1) layer of cotton dress shirt (3.5 ounces/yard) and (1) cotton T-shirt (5.25 ounces/yard) layered firmly on the face of the ballistic gelatin block.
The 4 layers of denim standard is from the IWBA in 1998 and was developed by Duncan MacPherson because the California Highway Patrol kept having bullets that passed FBI standards fail to expand in the real world.
For a more detailed history see: http://thinlineweapons.com/IWBA/1998-Vol3No3.pdf