This is another version from the Brits in Portugal Facebook Page, which might be a little easier to understand.
SEF has started to notify British nationals resident in Madeira or the Azores under the Withdrawal Agreement, today in order to schedule appointments with SEF to collect their biometric data for the new residency card. Only residents registered in the Brexit portal will be notified.
SEF and the British Embassy in Lisbon recommend that all British nationals resident in Portugal under the Withdrawal Agreement, register on the Brexit portal as soon as possible ➡ https://brexit.sef.pt/en/
This is a pilot project directed at residents on the islands. British nationals resident in mainland Portugal will be notified by SEF once phase 2 of this process starts.
You do not need to contact SEF regarding appointments, since all who are registered in the portal will be directly notified by SEF.
For those whose residency documents have or are expiring, the certificate with the QR code issued through the SEF portal is an official residency document that is valid until the new card is issued. You do not need to renew your residency if you are registered on the portal.
The British Embassy will continue to update you on this process, so please follow this page and our Living in Portugal guide ➡ https://www.gov.uk/guidance/living-in-portugal