A is the patentee. A sells the products produced by the production department to B, and the fees paid by B to A include patent fees.
Therefore, the patent right of this product has been exhausted in this transaction.
B is resold to C, C is resold to D, and so on, without paying patent fees.
In practice, it is only necessary to prove the legal source of the product. (Provide valid invoices and receipts. )