The Single Euro Payments Area (SEPA) currently consists of 36 countries, amongst them all 27 EU member states. SEPA's goal is to unify cashless payments in Euro amongst those within the area. A SEPA transfer works the same way as a domestic transfer. The country doesn't need to have EUR as a currency to take part in SEPA, as long as the transfer is done in EUR.
Of course, that means that if an international transfer even within the SEPA area is done in another currency than EUR, it automatically becomes a foreign transfer (non-SEPA). Also, transfers to countries outside of SEPA will always be foreign transfers (non-SEPA).
As of 2024, the following countries are part of SEPA:
Andorra
Austria
Belgium
Bulgaria
Croatia
Cyprus
Czech Republic
Denmark
Estonia
Finland
France (incl. overseas departments)
Germany
Greece
Hungary
Iceland
Ireland
Italy
Latvia
Liechtenstein
Lithuania
Luxemburg
Malta
Monaco
Netherlands
Norway
Poland
Portugal (incl. Azores and Madeira)
Romania
San Marino
Slovakia
Slovenia
Spain (incl. Canary Island, Ceuta and Melilla)
Sweden
Switzerland
Vatican
United Kingdom (incl. Gibraltar, Guernsey, Jersey, Isle of Man)
Please note that Greenland and the Faroe Islands are not part of the SEPA area. This also applies to Kosovo and Montenegro: though they do have the EURO as their countries' currency, they are not part of the SEPA area.