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

Code Definition
SHALL Support for the specified capability is required to be considered conformant.
SHOULD Support for the specified capability is strongly encouraged, and failure to support it should only occur after careful consideration.
MAY Support for the specified capability is not necessary to be considered conformant, and the requirement should be considered strictly optional.
SHOULD-NOT Support for the specified capability is strongly discouraged and should occur only after careful consideration.
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