Hi,
The control happens always on the amount level. However, the commitment reduction could be managed on the basis of a quantity. For example, PO with 5 units of 10$ each is reduced by a IR of 2 units with a price of 12$. If the commitment is managed on amount level the reduction would be 24, hence the rest of commitment would be 50-24=26. But, if the commitment is managed on quantity level, the reduction would be only 20 (2*10), and the commitment will remain on the level of 30. This is defined in the unit of measurement of the article in question.
I emphasize once more, though, that all the controls are happening on the amount level only.
Regards,
Eli