There are significant differences between unions and free trade zones. Both types of trading blocs have internal agreements that the parties enter into to liberalize and facilitate trade between them. The key difference between unions and free trade zones is their approach to third parties [lack of ambiguity needed]. While a customs union requires all parties to apply and maintain identical external tariffs on trade with non-parties, parties to a free trade area are not subject to such a requirement. Instead, they can set and maintain any customs regime for imports from non-parties, as they see as necessary.  In a free trade area without harmonized external tariffs, the parties will adopt a system of preferential rules of origin to eliminate the risk of trade diversion [necessary ambiguities].  24. Sopranzetti S. Free trade agreements and international trade: a network approach.
World Econ. (2018) 41:1549-66. doi: 10.1111/twec.12599 The agreement reflects the World Organization for Animal Health`s (OIE) negligible classification of risks for bovine spongiform encephalopathy (BSE) in the United States. It should be noted that with regard to the qualification of the original criteria, there is a difference in treatment between inputs originating and outside a free trade agreement. Inputs originating from a foreign party are normally considered to originate from the other party when they are included in the manufacturing process of that other party. Sometimes the production costs generated by one party are also considered to be those of another party. Preferential rules of origin generally provide for such a difference in treatment in determining accumulation or accumulation. This clause also explains the impact of a free trade agreement on the creation and diversion of trade, since a party to a free trade agreement is encouraged to use inputs from another party to allow its products to originate.
 First, tariffs and other rules applicable to trade with non-parties to this free trade area in each of the parties that signed a free trade area in force at the time of the creation of this free trade area must not be higher or more restrictive than tariffs and other rules applicable in the same signatory countries prior to the creation of the free trade area.