Production outsourcing services fields

I’m trying to figure out how production_outsoursing works, I stopped on supplier_service_supplier field which is not explained in the docs and also the help attribute it’s so much similar to supplier_service.
supplier_service = The service to buy to the supplier for the production
supplier_service_supplier = The supplier's service to buy for the production

Which are the diferences between those fields? Which is the use case?
Once I understand I will submit a patch to improve those help attributes.

It is the same difference as between the product and the product_supplier on the purchase line.
It is just the precision of which service for the supplier is used. When the supplier_service is the service defined by the company.