Standard UK usage for hospital visits is out-patient, in-patient and day case. It is easy to tell the latter two from the first because you are given an identifying wrist bracelet with a hospital number on it (a few years ago I had a diagnostic test where the machine used was located in the day hospital and I had to be formally admitted as a patient, even though it was only for 30 minutes).
I think that ambulatory care may not only cover out-patients, but also other healthcare services provided by, for instance, GP practices, or in the US HMOs. The key thing for typical out-patient attendance is that the care is usually under a named consultant (from a given specialty). An out-patient visit might be part of a single “episode of care” (e.g., maternity services are a complex mixture of clinic visits, hospital stay etc) or a standalone episode.
Some hospital clinics may offer non-consultant led services for long-term conditions: for instance, ear cleaning (wax removal etc.) was provided at my local hospital.
I no longer have any idea if there is a good open data framework for healthcare in Europe which might help, although there were certainly initiatives 30 years ago and the NHS model is a very rich one which is always worth consulting.