The EHIC can be used to cover any necessary medical treatment due to either an accident or illness within the European Economic Area and Switzerland. The card entitles the holder to state-provided medical treatment within the country they are visiting and the service provided will be the same as received by a person covered by the country's 'insured' medical scheme. This may not cover all of the services you would expect within the UK and you may have to make a contribution towards the care you receive.

The EHIC can also be used to receive treatment for pre-existing illnesses and chronic diseases, but conditions do apply so please check with your healthcare provider before you travel. Maternity care is covered by the EHIC whilst you are away, but if you are travelling to a country specifically to have a baby then you will need to complete an E112 form. Again, ask your healthcare provider for more information before you travel.

The EHIC may not cover persons for all medical costs incurred, so you are strongly advised to also arrange travel insurance to ensure that you are covered for all possible eventualities. Furthermore, you will not be covered by an EHIC if the main purpose of your travel is to receive medical treatment.

The EHIC is valid within the European Economic Area, which includes the European Union countries, Iceland, Liechtenstein and Norway. Switzerland also has an agreement in place with the European Union to accept the EHIC.

Whether there's any cross-border billing behind the scenes, I don't know.  If so, it raises an interesting question, because when my mother had a minor heart attack in Germany some years ago, she received a local standard of care: far better and more costly than she would have received at home. I know that the UK and Australia have a reciprocal agreement (We'll look after yours if you'll look after ours), and, if it's restricted to emergencies only, I'd imagine this was more efficient than running a billing bureaucracy.

There may also be individual agreements between EU countries.  The UK, for instance, has a pre-EU agreement with Ireland where it pays a substantial annual amount into the Irish healthcare system to compensate for the Irish citizens who paid tax all their working lives in the UK, and then retired to Ireland.