Phil,
It is called "free flow" and that design allows fluid to flow underneath. It is expensive, but in my case, I didn't have to pay for it all. My son lives in NC and bought a large amount off Craigslist - maybe 14-1500 square feet of both new and used from a company that set up NASCAR displays of show cars for various teams. He did his 3 car garage and brought the rest up to me. The remainder did almost 1/2 of my 936 square ft. garage. I called race deck and was able to negotiate a better price for the rest with free shipping that I needed. Has pros and cons. I have to use a shop vac to vacuum all the slots out, It is hard on the knees - I have to have a car mat down. The stuff is tough though. I have pulled and installed several motors with my cherry picker not wrecking it. I did cut into a couple of tiles with jack stands. You can snap it all apart, clean the floor up if you want and put it back down. Goes down in a couple of hours so very easy to install. It is shipped in 2'X2' boxes (about a foot high?) so can come regular shipping like UPS.