This code system http://hl7.org/fhir/focus-characteristic-code defines the following codes:

CodeDisplayDefinition
citation CitationUsed to reference a specific article.
clinical-outcomes-observed Observed outcomes are clinical outcomesUsed to denote a focus on clinical outcomes, ie evidence variable in role of outcome (measured variable) as observed is considered a "clinical outcome" (patient-important outcome such as mortality, symptoms, function or quality of life).
population PopulationThe population of interest.
exposure ExposureThe exposure of interest, such as an intervention.
comparator ComparatorThe comparator (intervention or control state) of interest.
outcome Outcomethe outcome of interest.
medication-exposures Medication exposuresAny medication exposures. A subset of exposures or interventions that are medications.
study-type Study typeType of research study, such as randomized trial or case-control study.
<status value="active"/><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="Evidence focus characteristic code."/><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://hl7.org/fhir/ValueSet/focus-characteristic-code"/><content value="complete"/><concept><code value="citation"/><display value="Citation"/><definition value="Used to reference a specific article."/></concept><concept><code value="clinical-outcomes-observed"/><display value="Observed outcomes are clinical outcomes"/><definition value="Used to denote a focus on clinical outcomes, ie evidence variable in role of outcome (measured variable) as observed is considered a "clinical outcome" (patient-important outcome such as mortality, symptoms, function or quality of life)."/></concept><concept><code value="population"/><display value="Population"/><definition value="The population of interest."/></concept><concept><code value="exposure"/><display value="Exposure"/><definition value="The exposure of interest, such as an intervention."/></concept><concept><code value="comparator"/><display value="Comparator"/><definition value="The comparator (intervention or control state) of interest."/></concept><concept><code value="outcome"/><display value="Outcome"/><definition value="the outcome of interest."/></concept><concept><code value="medication-exposures"/><display value="Medication exposures"/><definition value="Any medication exposures. A subset of exposures or interventions that are medications."/></concept><concept><code value="study-type"/><display value="Study type"/><definition value="Type of research study, such as randomized trial or case-control study."/></concept></CodeSystem>