D basically means it will run through all gears by it self. D3 means that City having a 5 speed AT has 5 gears. In D3 it will not shift above the 3rd gear. D2 would limit it to 2nd Gear and D1 would limit it to first gear only. For everyday drive always use D only. D3,D2,D1 is only used when descending from a hill or climbing a hill when you need to keep the car in 2nd or 3rd gear max
