Executed Writing = "Legal instrument is a legal term of art that is used for any formally executed written document that can be formally attributed to its author, records and formally expresses a legally enforceable act, process, or contractual duty, obligation, or right, and therefore evidences that act, process, or agreement." https://legal-dictionary.thefreedictionary.com/execute https://en.wikipedia.org/wiki/Legal_instrument
They are speaking of a Contract--specifically, they are speaking of an Executed (completed) Contract (writing).
They are saying that the Supplier is responsible for its subcontractors. Meaning that if the Supplier does not do the work itself, it is still responsible for any failure in quality, quantity, pricing, delivery, or anything related to what the Supplier gives its Customer--although the Supplier is acting as an intermediary.