This code system http://terminology.hl7.org/CodeSystem/variable-role defines the following codes:

Code Display Definition
population population variable represents a population.
subpopulation subpopulation variable represents a subpopulation.
exposure exposure variable represents an exposure.
referenceExposure reference exposure variable represents a reference exposure.
measuredVariable measured variable variable represents a measured variable.
confounder confounder variable represents a confounder.
<status value="draft"/> <experimental value="false"/> <date value="2020-12-28T16:55:11+11:00"/> <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="The role that the assertion variable plays."/> <caseSensitive value="true"/> <valueSet value="http://hl7.org/fhir/ValueSet/variable-role"/> <content value="complete"/> <concept> <code value="population"/> <display value="population"/> <definition value="variable represents a population."/> </concept> <concept> <code value="subpopulation"/> <display value="subpopulation"/> <definition value="variable represents a subpopulation."/> </concept> <concept> <code value="exposure"/> <display value="exposure"/> <definition value="variable represents an exposure."/> </concept> <concept> <code value="referenceExposure"/> <display value="reference exposure"/> <definition value="variable represents a reference exposure."/> </concept> <concept> <code value="measuredVariable"/> <display value="measured variable"/> <definition value="variable represents a measured variable."/> </concept> <concept> <code value="confounder"/> <display value="confounder"/> <definition value="variable represents a confounder."/> </concept> </CodeSystem>