Rules of origin are particularly important in free trade agreements, which are intended to focus exclusively on products of preferential origin. In this context, rules of origin are essential to distinguish between products originating from contracting parties and those from third countries. This differentiation is intended for two purposes: (1) it allows the importing party to determine whether a product can benefit from preferential treatment under the independent free trade agreement; (2) it avoids the scenario in which exports from third countries enter the free trade agreement through the lowest external tariff member (i.e. trade transfer).  This explains why, in a customs union, it is not necessary to establish rules of origin between their contracting parties – members of a customs union are required to maintain a common external tariff rate on imports from third countries.  h) at the request of an exporter, importer or person with a legitimate reason, the original assessments they would assign to something are issued as soon as possible and no later than 150 days (3) after the request for such an assessment, provided all the necessary information has been provided. Applications for assessment are accepted before the start of trade in the property concerned and may be accepted at a later date. These assessments remain valid for three years, provided that the facts and conditions, including the rules of origin, are comparable. If the parties concerned are informed in advance, these assessments are no longer valid when a decision contrary to the assessment is made as part of a review in accordance with point (j). These assessments are made available to the public, subject to the provisions of point (k); Among the three methods of applying the „substantial transformation“ test, changing the tariff classification in the Rules of Origin Agreement is considered a primary method. In Article 9 on the objectives and principles of harmonization of rules of origin, the original agreement divides „substantial transformation“ into two groups in which the „change in tariff classification“ differs, while other methods are considered „complementary“. This article states that the harmonization program to ensure the timely completion of the harmonization program is „implemented on the basis of a product sector, as presented by different chapters or sections of the harmonized system nomenclature. It is only when the use of the nomenclature does not allow the correct expression of a „substantial transformation“ that the technical committee of the rules of origin envisages „developing in a complementary or exclusive manner the use of other requirements, including percentages of value and/or manufacturing or processing operations“.
 With respect to certification, a distributor must know whether or not self-certification is authorized by the trade agreement under which it invokes preferences.