Question regarding End Item Substitute functionality in DPP

Hi All

we are checking the option to implement DPP in our Org and we have a Quseation regarding End item Substitute which we got from our busness

will appricate your ansawer :

Primary Item A has substitute item B

Item A has forecast In ORG X

Item A in Org X source from Org X1 (Regional Hub)

item A in Org X1 Source from Org X2 (Central Hub)

Item A in Org X2 source item from external supplier

The question is in case there is no supply at all from item A in all above source chains while in Org X2 there IS exist supply (On hand ) from substitute item B,