It is an ergonomic topper that you put on top of your mattress to get better and more focused support. 7-zone comforter has 3 main zones which differ in firmness in response to the body part it supports. Lumbar zone is thicker to fill in the lower back arc, hip zone is softer to compensate the curve of your hip, and leg zone is enhanced so that it lift the leg a little for muscle relaxation. Because mattress is flat but we are not. We have curves. Differences of the firmness within the zones of 7-zone comforter may not be much but it makes a lot of difference in the comfort you experience.