This code system http://terminology.hl7.org/CodeSystem/service-referral-method defines the following codes:

CodeDisplayDefinition
fax FaxReferrals may be accepted by fax.
phone PhoneReferrals may be accepted over the phone from a practitioner.
elec Secure MessagingReferrals 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.
semail Secure EmailReferrals may be accepted via a secure email. To send please encrypt with the services public key.
mail MailReferrals may be accepted via regular postage (or hand delivered).
 <status value="draft"/> <experimental value="false"/> <date value="2020-04-10T07:10:28+10:00"/> <publisher value="Patient Administration"/> <contact> <name value="Patient Administration"/> <telecom> <system value="url"/> <value value="http://www.hl7.org/Special/committees/pafm"/> </telecom> </contact> <description value="The methods of referral can be used when referring to a specific HealthCareService resource."/> <jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <caseSensitive value="true"/> <valueSet value="http://terminology.hl7.org/ValueSet/service-referral-method|1.0.1"/> <content value="complete"/> <concept> <code value="fax"/> <display value="Fax"/> <definition value="Referrals may be accepted by fax."/> </concept> <concept> <code value="phone"/> <display value="Phone"/> <definition value="Referrals may be accepted over the phone from a practitioner."/> </concept> <concept> <code value="elec"/> <display value="Secure Messaging"/> <definition value="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."/> </concept> <concept> <code value="semail"/> <display value="Secure Email"/> <definition value="Referrals may be accepted via a secure email. To send please encrypt with the services public key."/> </concept> <concept> <code value="mail"/> <display value="Mail"/> <definition value="Referrals may be accepted via regular postage (or hand delivered)."/> </concept> </CodeSystem>