E9.2 Cross Ref (P4104) Replacement Items for BOMs


I am trying to understand the correct way of using P4104 (cross refs) with "replacement" items. Specifically I am trying to understand how / when I should be replacing the old item with the new in my BOMs.

As toy example, initially we have a BOM defined for a product SKU-FINISHED.
That BOM has one input:

We then use P4104 to define a cross ref “replacement” for SKU-INPUT-A: SKU-INPUT-B and we set the stocking type for SKU-INPUT-A to be `U` (use up).

I am trying to understand the difference between these two scenarios:
  1. We update the BOM to have SKU-INPUT-B
  2. We leave the BOM as is with SKU-INPUT-A

What will happen in either of those cases when:
  1. There is inventory of only SKU-INPUT-B
  2. There is inventory of only SKU-INPUT-A
  3. There is inventory of both SKU-INPUT-A and SKU-INPUT-B

Specifically I am curious as to (for each of those three scenarios):
  • Which input will be used in BOMs associated with WOs generated by MRP?

And then if there were no inventory in either, what ordering messages will be generated to procurement (i.e. to order either / or SKU-INPUT-A or SKU-INPUT-B?

Ultimately, I want to understand am I supposed to update the BOMs as soon as we set up the cross-ref? Or should I wait to update the BOM until we have entirely used up the inventory of SKU-INPUT-A? And are there pros / cons as to when I do the update?