A HD closure is a hair piece that's installed at a wider section of the hairline than the 4X4 closure, along with 2-3 bundles of hair. 
It is used when you want to create the illusion of a natural part or hair line and are most popular for women who have traction alopecia, thinning edges, or desires a protective style.
The 5X5 HD closures have smaller knots and thinner lace, no shedding and  tangeling and may be dyed to #27 (level 8). The hair tone is natural brown , at a level 2 or 3.