A claim is the moment an accrued entitlement becomes a demand. On the receivable side, claims you raise against suppliers must be filed inside the agreement’s window, in the supplier’s required format, with the required support. On the payable side, claims submitted to you must be validated before payment.
Both directions have the same underlying requirement: a link between the claim and the agreement or authorisation that justifies it. Where that link is manual, the receivable side under-claims and the payable side over-pays.