The REX system is a particular case of invoice declarations, based on a principle of self-certification by economic operators who issue themselves so-called ‘statements on origin’.
A statement on origin is a declaration of origin added by the registered exporter on the invoice or any other commercial document. The text of the statement on origin is given in Annex 22-07 of the UCC IA (Commission Implementing Regulation (EU) 2015/2447 (OJ L-343 29/12/2015) (CELEX 32015R2447).
To be entitled to make out a statement on origin, economic operators have to be registered in a database by the competent authorities of their country of origin. Following this registration the economic operator will become a ‘registered exporter’.
Information on the registered exporter's data is published on the EU REX website where economic operators are able to verify the validity of the registrations of the registered exporters who submit statements on origin.
Note that for consignments of a value less than €6,000, the statement of origin can be made with no obligation for registration.
Progressive implementation of the REX System
The REX system is progressively replacing the current system based on certificates of origin issued by governmental authorities and on invoice declarations made out by economic operators.
See its current state of implementation and the list of countries applying it here.