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.
Tariffication
The main economic issues that arise with tariffication stem from the nonequivalence of tariffs in NTBs in a number of scenarios. The issue analyzes nonequivalence arising from the existence of imperfect competition in importing countries, price instability in importing and exporting countries, and inefficient allocation of quantitative restrictions. It is shown that in all these cases the definition of an appropriate "equivalent tariff" to be used in tariffication is not straightforward, and that in general this equivalent tariff cannot be computed on the basis of only observed price differences between countries. Tariff-rate quotas, which are meant to be the main tool in implement tariffication according to the existing proposal, are analyzed in some detail. Concerning the relationship between tariffication and the other elements of the trade liberalization package, it is shown that tariffication would limit the scope of export subsidy policies, and that the existence of production and export subsidies makes observed price gaps between countries of questionable value in setting equivalent tariff levels. Finally, it is argued that the main focus on tariffication should be the conversion of NTBs to acceptable long-run (bound) tariffs rates, and considerable flexibility in the conversion process could be exercised during the transition period.