The Long-term customer declaration (LTCD) is a document issued by the supplier to confirm the preferential origin of goods in line with the corresponding rules of origin outlined in the Preferential Trade agreement. These requirements (list of rules) are listed on the EU Commission website: https://ec.europa.eu/taxation_customs/customs/customs_duties/rules_origin/preferential/index_en.htm.
The legal basis is the Commission Implementing Regulation (EU) 2015/2447 of 24 November 2015 laying down detailed rules for implementing certain provisions of Regulation (EU) No 952/2013 of the European Parliament and of the Council laying down the Union Customs Code. According to the Regulation (EU) No 952/2013 (UCC) it is possible to issue one Long-term declaration for future and past periods.
The text of the declaration is legally binding according to the regulation mentioned above. It is only valid if the signer is resident in the EU. The issuer is responsible for the correctness of the declaration provided towards the recipient and the Customs Authority. Therefore, the issuer shall inform the recipient immediately where the LTCD is no longer valid in relation to the goods provided.