Assuming the commenter is referring to the example binding strength for Observation.bodysite The scope of US Core is broad and hence only a limited set of elements are defined for broad interoperability.   The bodysite element is optional in all the Observation profiles and therefore it's valueset binding defaults to the base Observation resource.  For particular use cases such as ICU it can represented using this element and any codesystem by an implementer or by a implementation guide that further constrains these profiles to fit a narrower us case.   Note that bodysite can also be represented in the Observation code when using LOINC.