Sub-floor is used with dance floors when the dance floor is being set up on grass or dirt which is not perfectly level. The sub-floor includes a ground tarp, plywood sheets, and wood chips as necessary to fill in any low spots in your ground. The price is per 3’x3′ of sub-floor needed. Adding sub-floor does not guarantee good stability or appearance of the dance floor. That is all dependent on the area which you are setting the dance floor upon.