The gear is kind of more related to ground speed. You start in what ever get you want not like an auto transmission where you go up in gears. There is less torque to pull a ground engaging implement the faster you go. But if you find the torque is adequate and you can plow at 9mph go for it!!! Haha. Disks do better if your going faster than slower.
You won't be able to bushhog in the fastest gear with anything over a 4ft bh and short grass. But it will beat you to death going wide open.
The L side is so slow I almost never use it, it's good for tilling and close maneuvers but not much more.