This code system http://hl7.org/fhir/research-study-objective-type defines the following codes:

CodeDisplayDefinition
primary PrimaryThe main question to be answered, and the one that drives any statistical planning for the study—e.g., calculation of the sample size to provide the appropriate power for statistical testing.
secondary SecondaryQuestion to be answered in the study that is of lesser importance than the primary objective.
exploratory ExploratoryExploratory questions to be answered in the study.
<status value="active"/><experimental value="false"/><date value="2022-05-15T16:55:11.085+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="Codes for the kind of study objective."/><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/research-study-objective-type"/><content value="complete"/><concept><code value="primary"/><display value="Primary"/><definition value="The main question to be answered, and the one that drives any statistical planning for the study—e.g., calculation of the sample size to provide the appropriate power for statistical testing."/></concept><concept><code value="secondary"/><display value="Secondary"/><definition value="Question to be answered in the study that is of lesser importance than the primary objective."/></concept><concept><code value="exploratory"/><display value="Exploratory"/><definition value="Exploratory questions to be answered in the study."/></concept></CodeSystem>