That is up to the holder – you can choose from either. Both contain a “digitally signed QR code” which can be scanned on entry to a country. Member states have agreed on a common design. It contains all the same information as a typical vaccine passport such as supplier and batch number, and can also store data about test results and previous history of COVID-19 infections/recovery. These certificates are available free of charge and available in both the member countries’ official language as well as English. Each issuing body (e.g. a hospital, a test centre, a health authority) has its own digital signature key. All of these are stored in a secure database in each country.