This case-sensitive code system http://hl7.org/fhir/us/ndh/CodeSystem/HealthcareServiceReferralMethodCS defines the following codes:

Code	Display	Definition
ccda	CCDA	CCDA with endpoints
elec	Secure Messaging	Referrals may be accepted via a secure messaging system. To determine the types of secure messaging systems supported, refer to the identifiers collection. Callers will need to understand the specific identifier system used to know that they are able to transmit messages.
electronic-referral	Electronic Referral	Referrals may be accepted via Electronic Referral endpoints
electronic-scheduling	Electronic Scheduling	Referrals may be accepted via Electronic Scheduling endpoints
fax	Fax	Referrals may be accepted by fax.
phone	Phone	Referrals may be accepted over the phone from a practitioner.
mail	Mail	Referrals may be accepted via regular postage or hand delivered.
semail	Secure Email	Referrals may be accepted via a secure email. To send please encrypt with the services public key.