The Registered Exporter system (the REX system) is a system of certification of origin of goods based on a principle of self-certification.The origin of goods is declared by economic operators themselves by means of so-called statements on origin. To be entitled to make out a statement on origin, an economic operator has to be registered in a database by his competent authorities. The economic operator becomes a “registered exporter”.
The REX system is the term used to designate the system of certification of origin as a whole, and not only the underlying IT system which is used for the registration of exporters.

For the time being, the REX system is used by EU exporters in the context of some FTAs, in the context of the GSP of the EU and in the context of the Overseas Association Decision.
The REX IT system has been developed by the European Commission and is made available to the Member States of the EU, to the GSP beneficiary countries and to the OCTs. It takes the form of a Web application accessed with a username and a password as a website through Internet. The only technical requirement is then to use a device connected to Internet to be able to use the REX system.   

(a) On the GSP Origin Rules:

https://ec.europa.eu/taxation_customs/business/calculation-customs-duties/rules-origin/general-aspects-preferential-origin/a-guide-users-gsp-rules-origin_en

   (b) On the REX System:

https://ec.europa.eu/taxation_customs/business/calculation-customs-duties/rules-origin/general-aspects-preferential-origin/arrangements-list/generalised-system-preferences/the_register_exporter_system_en