42 C.F.R. § 485.502
As used in this subpart, rural emergency hospital or REH means an entity that operates for the purpose of providing emergency department services, observation care, and other outpatient medical and health services specified by the Secretary in which the annual per patient average length of stay does not exceed 24 hours. The time calculation for determining the length of stay of a patient receiving REH services begins with the registration, check-in or triage of the patient (whichever occurs first) and ends with the discharge of the patient from the REH. The discharge occurs when the physician or other appropriate clinician has signed the discharge order, or at the time the outpatient service is completed and documented in the medical record. The entity must not provide inpatient services, except those furnished in a unit that is a distinct part licensed as a skilled nursing facility to furnish post-hospital extended care services.
42 C.F.R. §485.502