EU-UK FTA: What is exporter reference No? (Part I)

Zero duties in the EU-UK trade will apply only to originating products. In most cases the proof of origin will be the statement on origin made out by the exporter in a commercial document. Reader’s questions: „There is an Exporter Reference Number indicated in the form of the statement on origin. When and how should an EU exporter obtain it? How long does it take? May the exporter issue the statement on origin after exportation of the goods?“.