Reservations & Marking

Can someone describe these features to me?  We are  having issues with this in our implementation of ax 2012 as it seems new production orders are reserving their materials and then purchase receipts cannot be completed due to the reservation.

Also, when removing the reservation I see that there is often a 'marking' for the same quantity that was previously reserved that I have to remove to allow purchase orders to be received.

Any info or insight into these functions is greatly appreciated.