A more thorough explanation:

Definition: Unascertained duty refers to a preliminary, estimated payment made to a customs collector for the duty that will be due on final accounting. It is a duty that is not yet determined or identified.

Example: An importer wants to bring in a shipment of goods from another country. The customs collector requires the importer to pay an unascertained duty as a preliminary payment before the goods can be landed and sold. The final amount of duty owed will be determined after the goods are assessed and valued.

Explanation: The example illustrates how an unascertained duty is a preliminary payment made to a customs collector before the final amount of duty owed is determined. It is an estimated payment made to allow the goods to be landed and sold while the final amount of duty is being assessed and valued.