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.
Feed-in tariff
A feed-in tariff can differentiate on the basis of marginal cost. This is a theoretical alternative which is based on the concept of price differentiation (Finon). Under such a policy the tariff price ranges from some level slightly above the spot rate to the price required to obtain the optimal level of production determined by the government. Firms with lower marginal costs receive prices on the lower end of the spectrum that increase their revenue but not by as much as under the uniform feed-in tariff. The more marginal producers face the higher tariff price. This version of the policy has two objectives. The first is to reduce the profitability of certain production sites.