Some disciplines have modifiers on the procedure that specify that the procedure was in-hospital. eg OT 0021, PT 0014 etc which were historically used to indicate an in-hospital claim.
However it seems that some medical aids now want you to have a place of service entered on the invoice. You can set this functionality on in Setup - System Variables - Invoicing.
Then on each invoice you can select from Hospital, Rooms and othere locations. Check the Helpfiles for a full list of possible locations including Rehab Centres, clinics etc