Full Tilts, Dalbello, and Roxa use a 3 piece design for their boots so the boot is split into three parts, the shell, cuff and tongue which are then connected together. The main advantage to the three piece design is it produces a very smooth and consistent flex which should match the flex of your ankle more naturally compared to traditional overlap/normal boots. Also you can change the tongue on most 3-Piece boots which allows you to adjust the flex of the boot, stiffer tongue = stiffer boot and vice versa.
However buy what fits you whether it be a traditional overlap boot or a 3-Piece. Go to a good boot fitter and try on what is given to you. Make sure you get custom footbeds made as well. Neither design is superior to the other just choose whichever design works best for your legs and feet.