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-rate quota
A tariff-rate quota (TRQ) is a two-tiered international trade tariff that combines two policy instruments used historically to protect domestic production by restricting imports−Import quotas and tariffs. The quotas set a quantitative threshold for a designated time period for imports of a protected domestic product. Under that threshold, tariffs applied to imports are either very low or nonexistent. Over that threshold, imports are subject to a significantly higher tariff that is intentionally prohibitive. TRQs have been required by the World Trade Organization (WTO) since the 1995 Uruguay Round Agreement 123 contract party nations engaged in international trade.