The issue of becoming an only representative is a question of mutual agreement between the "non-EU manufacturer" and the natural or legal person established in the EU who is being appointed as an only representative.
"Non-EU manufacturers" need to send a letter confirming this appointment to their only representative who must have it available in case of inspection by the relevant Member State's enforcement authority. No such letter has to be sent to ECHA. However, when compiling the registration dossier in IUCLID 5 the only representative is advised to attach this letter of appointment to the registration dossier in the field "Official assignment from non EU manufacturer" in section 1.7.
In addition the "non-EU manufacturer" shall inform the importer(s) within the same supply chain of the appointment of the only representative according to Article 8(3) of the REACH Regulation. These importers shall be regarded as downstream users.