The Registered Exporter system (the REX system) is a system of certification of origin of goods based on a principle of self-certification.
This system is gaining wider recognition with Brexit as EU companies need to be “REX registered” to issue origin statements which allow for exports of goods over
€ 6000 to benefit from zero or preferential import tariffs when entering the UK.
The Portuguese Customs Authorities have published guidelines on REX within the framework of the EU-UK Trade and Cooperation Agreement.
The guidelines are available here [in PT version]Request Info