To foster interoperability the LevelofEvidence Component should have a preferred binding to a ValueSet. In particular to Clinical Annotation Levels of Evidence.

As http://build.fhir.org/terminologies.html#preferred states:
Instances are encouraged to draw from the specified codes for interoperability purposes but are not required to do so to be considered conformant.

Preferred bindings are used when there is consensus at the specification level about the coded values that are the best to be used, but there is recognition that some implementation contexts are unable to use the recommended codes for a variety of reasons. Applications should consider adopting the preferred value set wherever possible, as these preferred value sets are the most likely to serve interoperability purposes in the future.