Hi,
The overall limit represents the maximum value that the total of all unplanned services (or the value of the material) item may not exceed. Its for control purpose.
If you are using item categary B, the value you are giving in "Expected Value" will replace the net value in the PO. So the overall limit will be on net vale (which is same as expected value)
In case of item category D, the net price will be the total of expected value and the item level price foir the service. Still you can restrict the max limit by usimg the overall limit value.
Hope its helpful.
Regards,
AKPT