SMP lever on 5711?

Hi,
Below the right hand side of the seat on my 5711,theres a lever with 3 settings-S,M and P, I think. Could anyone tell me what the settings mean please? And how/when they should be used?
Many thanks.

D= draft control , for heavier soil acting implements, main lever is either up or down with the lower half of its travel being the weight transfer and depth control
M= mixed draft and position, for light soil acting implements whose depth is directly controlled by the position main lift lever, but is also affected by the movement of the top link.
P= position control, height of the arms is directly proportional to the position of the lift lever with no contribution from the top link.

Thats great-many thanks for that Dave! I thought it had somthing to do with the lift arms,but now I know exactly what.
Thanks again.