Portugal provides double taxation agreements for businesses so that they don't pay double tax. This article explains why these agreements do not apply to Cloudbeds customers.
The DTA documents listed here http://info.portaldasfinancas.gov.pt/pt/apoio_contribuinte/modelos_formularios/double_tax_conventions/ are provided by the Portugal government to avoid properties from paying double taxes over the commissions.
The Mod 21 document is issued by Booking.com, since Booking.com collects taxes on commissions on behalf of their headquarters in the country of Amsterdam (not Portugal). The taxes remain in the EU, but Portugal needs that proof that the taxes were already collected.
As there is an agreement in the European Union, the taxes are collected in Amsterdam and remain in Amsterdam, even though the services were executed in Portugal.
The Mod. 21 is a way to avoid double taxation (Amsterdam and Portugal for the same service) and also to avoid properties around the EU to say they payed taxes somewhere else while they never paid taxes anywhere in the EU.
Since Cloudbeds does not collect taxes in the EU, and since we do not charge commissions on bookings, the Mod 21 does not apply to our customers for the purposes of Cloudbeds fees.
If you are in need of a DTA document for tax purposes related to the commissions paid to your OTA channels, please contac them directly (the OTA channels) to provide the DTA document to you. There is no such document for Cloudbeds services, since we do not charge commissions, and we do not pay taxes to the EU.