Economists have tried to assess the extent to which free trade agreements can be considered public goods. First, they deal with a key element of free trade agreements, the system of on-board tribunals, which act as arbiters in international trade disputes. These serve as a clarification of existing statutes and international economic policies, as confirmed by trade agreements.  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].  The creation of free trade zones is seen as an exception to the nation`s most privileged principle (MFN) in the World Trade Organization (WTO), since the preferences that parties to a free trade area accord exclusively to each other go beyond their membership obligations.  Although GATT Article XXIV authorizes WTO members to establish free trade zones or to conclude interim agreements necessary for their establishment, there are several conditions relating to free trade zones or interim agreements leading to the creation of free trade zones.
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.  In general, trade diversion means that a free trade agreement would divert trade from more efficient suppliers outside the zone to less efficient suppliers within the territories.