Inpatient care refers to any treatment where the patient is required to be admitted to a hospital or health care facility facility. On the other hand, the OECD defines outpatient care as care that "comprises medical and ancillary services delivered to a patient who is not formally admitted to a facility and does not stay overnight." Thus, studies like this try to break out "inpatient and outpatient care" as an attempt to represent the "real" service in question, while isolating things like administrative costs.
I think outpatient care mean you don't stay in the hospital overnight. So you could have a surgery and leave within an hour in the recovery area and that is outpatient.