The term compound tariff can be used to describe an import fee made up of two different charges. The first charge could be based on a percentage of the commodity value, typically referred to as an ad valorem duty. The second charge could be a fixed amount per unit.
As an example, a commodity would be considered to have a compound tariff if it had an ad valorem duty of 5% and a $2 per unit fee. Few U.S. tariffs are structured this way, with most being either one or the other.
In the event that multiple ad valorem type duties or tariffs are applied, it would not technically be considered a compound tariff, just a layering of ad valorem duties. Such practices can be referred to in multiple ways.
See Also: Tariff Stacking
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A Guide to U.S. Import Taxes: Duties, Tariffs, and Other Fees