The usable build plate size refers to the actual area on the build plate that can be used for printing, while the size of the build surface refers to the total area of the build plate. The difference between the two values is due to the clips that attach the removable build plate to the print bed, which encroach upon the usable area of the build plate. The Creality Ender 3 has a build surface size of 235 x 235 mm, but the usable build plate size is 220 x 220 mm.