Work Definition Components to have Blank Source Subinventory

As per the understanding for the component items in the Work definition, the supply subinventory is the one from which the material is supplied. The hierarchy for defaulting the supply subinventory is as follows: work definition, item definition, plant parameter.

However, is there a way to keep this Supply Subinventory as blank?

The business case is: Consider a metal sheet as the raw material. This would be used for many Semi Finished(SF) and Finished good(FG) items and for those the supply source might wary from FG to FG or SF to SF. Therefore can it be blank at WD definitions?