Good question. I won't waste your time pretending to know the answer but I did a little digging and despite the pretence that this is 'Hard Coded' I could not see this used in the F4111 in Demo Data. What is more it is not set up in P40040 Doc Type Maintenance (but neither is IB so that doesn't mean a great deal!)
I enclose the White Paper on Weighted Average Cost as it might give you a clue although it says it uses "IB" in the Cardex as you would expect. I would be interested to see the answer to this.