This code system http://hl7.org/fhir/admit-source defines the following codes:

CodeDisplayDefinition
hosp-transTransferred from other hospitalThe Patient has been transferred from another hospital for this encounter.
emdFrom accident/emergency departmentThe patient has been transferred from the emergency department within the hospital. This is typically used in the transition to an inpatient encounter
outpFrom outpatient departmentThe patient has been transferred from an outpatient department within the hospital.
bornBorn in hospitalThe patient is a newborn and the encounter will track the baby related activities (as opposed to the Mothers encounter - that may be associated using the newborn encounters partof property)
gpGeneral Practitioner referralThe patient has been admitted due to a referred from a General Practitioner.
mpMedical Practitioner/physician referralThe patient has been admitted due to a referred from a Specialist (as opposed to a General Practitioner).
nursingFrom nursing homeThe patient has been transferred from a nursing home.
psychFrom psychiatric hospitalThe patient has been transferred from a psychiatric facility.
rehabFrom rehabilitation facilityThe patient has been transferred from a rehabilitiation facility or clinic.
otherOtherThe patient has been admitted from a source otherwise not specified here.