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

CodeDisplayDefinition
SHALL SHALLSupport for the specified capability is required to be considered conformant.
SHOULD SHOULDSupport for the specified capability is strongly encouraged, and failure to support it should only occur after careful consideration.
MAY MAYSupport for the specified capability is not necessary to be considered conformant, and the requirement should be considered strictly optional.
SHOULD-NOT SHOULD-NOTSupport for the specified capability is strongly discouraged and should occur only after careful consideration.
 <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="Indicates the degree of adherence to a specified behavior or capability expected for a system to be deemed conformant with a specification."/> <caseSensitive value="true"/> <valueSet value="http://terminology.hl7.org/ValueSet/conformance-expectation"/> <content value="complete"/> <concept> <code value="SHALL"/> <display value="SHALL"/> <definition value="Support for the specified capability is required to be considered conformant."/> </concept> <concept> <code value="SHOULD"/> <display value="SHOULD"/> <definition value="Support for the specified capability is strongly encouraged, and failure to support it should only occur after careful consideration."/> </concept> <concept> <code value="MAY"/> <display value="MAY"/> <definition value="Support for the specified capability is not necessary to be considered conformant, and the requirement should be considered strictly optional."/> </concept> <concept> <code value="SHOULD-NOT"/> <display value="SHOULD-NOT"/> <definition value="Support for the specified capability is strongly discouraged and should occur only after careful consideration."/> </concept> </CodeSystem>