This code system http://terminology.hl7.org/CodeSystem/expansion-parameter-source defines the following codes:

CodeDisplayDefinition
input Client InputThe parameter was supplied by the client in the $expand request.
server Server EngineThe parameter was added by the expansion engine on the server.
codesystem Code SystemThe parameter was added from one the code systems used in the $expand operation.
 <status value="draft"/> <experimental value="false"/> <date value="2020-04-09T21:10:28+00:00"/> <publisher value="HL7 (FHIR Project)"/> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <description value="Declares what the source of a parameter is."/> <caseSensitive value="true"/> <valueSet value="http://terminology.hl7.org/ValueSet/expansion-parameter-source"/> <content value="complete"/> <concept> <code value="input"/> <display value="Client Input"/> <definition value="The parameter was supplied by the client in the $expand request."/> </concept> <concept> <code value="server"/> <display value="Server Engine"/> <definition value="The parameter was added by the expansion engine on the server."/> </concept> <concept> <code value="codesystem"/> <display value="Code System"/> <definition value="The parameter was added from one the code systems used in the $expand operation."/> </concept> </CodeSystem>