Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Tariff
Customs duty is calculated on the determination of the "assessable value" in case of those items for which the duty is levied "ad valorem". This is often the transaction value unless a customs officer determines assessable value in accordance with the Harmonized System. For certain items like petroleum and alcohol, customs duty is realized at a "specific rate" applied to the volume of the import or export consignments.