material used by subcontractor in subcontracting process

I have an end product which require 5 sub materials to produce, 4 sub items are supplied by me and one subcontractor is using from his own.

I have a BOM for it. Where in BOM I can specifiy that the the item will be used by the subcontractor and will not be provided by me to him.

Can I specify this in PO if the BOM doesnot have the provision ?

I have to make it clear in the PO what will be provided by me and what will be used by subcontractor