Ok, I do not know the specifics about Yanmar but I do know about the different types of control.
Position Control is when you can set how high the TPH is. If you put the lever in the middle of it's travel, the TPH goes to the middle of it's movement up and down. You would normally use this for bush hogging or any other job when you need to set the height of the TPH.
Draft Control (The other lever) is for ground engaging attatchments. When you lower the lever, the TPH would normally drop to the floor. If you had a ground engaging attatchment on the TPH, like a plough, when you went to pull it along it would try to pull down deeper. The tractor is able to feel this force, either throught the top link, or lower link sensing (normally only on bigger tractors) and compensates for it by lifting the attatchment. This is a constant process and is always adjusting. By moving the draft lever, you can set how deep you want to attatchment in the ground. It will only work with ground engaging attatchments.
Sometimes, you have to have the draft control lever in it's upper or lower most position to use the position control.
Hopefully someone with a bit more knowledge of this tractor will tell you about this particular tractor.
Hope this helps, ask if you have any more questions.
Edit: You also have the little butterfly nuts above the levers. These are helpful to set the uppermost position of the TPH to prevent you from bending a pto shaft or something from lifting the mower too high.