I think the best place to begin is in the HIM-10 CMS Hospital manual. cms.hhs.gov/manuals/10_ho...0.asp#_1_5

(this would be section 210 COVERED INPATIENT HOSPITAL SERVICES)

cms.hhs.gov/manuals/10_ho....asp#_1_12

(this would be section 230.6 observation services)

Amy