When does "allocation" occur

I have recently been appointed to work with system 21 with in a large food manufacturer and our company suffers from a lot of negative stocks.
We have improved disciplines (i.e. making sure people book in on time), but still have a lot of issues.
We have work orders, that are first released and then become active and then complete at which point they backflush. 
Can somebody tell me when are stocks allocated to a work order, is it when an order is released or when it becomes active?, and what bearing either would have on negative stocks?

Thank you.
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