A physical inspection of goods is carried out pre-shipping, in the country of export, establishing the exact nature of the goods. The invoice and other documents are then scrutinized, and an accurate valuation, and customs tariff code, are assigned. These are used, in conjunction with the client country's published duty rates, to calculate the correct duties and taxes payable. The certificate is issued to the importer. This is used to substantiate the payment of full duty, prior to clearing the goods.
The actual duty collected is compared with the certificates, and any shortages can be investigated and corrected.