Why does the tariff system vary from country to country? Explain, giving examples of tariffs imposed by the United States with particular emphasis on preferential tariffs

What are the conditions under which GATT allows the use of preferential tariffs?

Tariff systems provide either a single rate of duty for each item applicable to all countries or two or more rates, applicable to different countries or groups of countries. The single-column tariff is the simplest type of tariff. It includes a schedule of duties in which the rate applies to imports from all countries on the same basis. Under the two-column tariff the first column includes "general" duties plus "special" duties indicating reduced rates determined by tariff negotiations with other countries. Rates agreed upon by "convention" are extended to all countries that qualify for normal trade relation (NTR). The United States have given NTR status to some 180 countries around the world. A preferential tariff is a reduced tariff rate applied to imports from certain countries. GATT prohibits the use of preferential tariffs, with three major exceptions. First are historical preference arrangements such as the British Commonwealth preferences and similar arrangements that existed before GATT. Second, preference schemes that are part of a formal economic integration treaty, such as free trade areas or common markets, are excluded. Third, industrial countries are permitted to grant preferential market access to companies based in less-developed countries.