DRP Logic

We are planning on generating a forecast from sales history to run DRP against.  Can anyone tell me how DRP knows where a product should be sourced from?  Does it look at the MCU value in the F42119 or the EMU value?  If it looks at the MCU value how do you tell it where the product should have been sourced from?  In other words, for some orders it was sourced from BP A but we need to plan on it being sourced from BP B.