You will receive care in accordance with the rules laid down in European Regulation 883/2004. This means that you usually don't have to pay the costs for the treatment yourself. Your health insurance company does. You must have given prior authorisation for this treatment. In that case, you will receive a document S2.
Treatment according to the legislation
You will receive the treatment in accordance with the legislation of the EU country where you are being treated. Because you are insured in the Netherlands, the health insurer does require that you have a reference letter. Just like in the Netherlands. Because here too, you need it for certain care. For example, for treatment by a medical specialist or in a hospital. You may also need to pay a contribution for the treatment to the healthcare provider.
The National Contact Point for Cross-Border Healthcare (NCP) of the country you want to go to can tell you which healthcare is covered by the statutory health insurance in that country. A list of NCPs can be found on the page 'Other National Contact Points'.
Also, your health insurer usually pays the costs if he has a contract with the foreign healthcare provider.