http://hl7.org/fhir/metadatatypes.html#Expression http://hl7.org/fhir/extension-valueset-expression.html

http://hl7.org/fhir/metadatatypes.html#Expression

When ValueSet.compose is not used, and the extension (http://hl7.org/fhir/extension-valueset-expression.html) is used to define an expression in place of the standard compose, the datatype is equal to Expression.

Expression.language is bound to (Extensible to http://hl7.org/fhir/codesystem-expression-language.html but limited to http://www.rfc-editor.org/bcp/bcp13.txt  )    (note: this url is for a text document)

This means that any code used for language must be either in the FHIR code system (expression language) or found in the text document. The document is not a code system.

To use another value, either the FHIR code system must be updated, or the document must be updated.

To remove confusion, and allow values such as SNOMED ECL, OWL to be used as an expression language, the following steps are proposed.

1. For the VSDP compliant profile on ValueSet, assign the Max finding to a new value set composed of bcp13 media type code system from IANA or V3 (https://www.iana.org/assignments/media-types/media-types.xhtml / https://terminology.hl7.org/CodeSystem-v3-mediatypes.html) , AND those found in the current FHIR Expession Language code system. 
2. Clarify the various code systems and value sets in FHIR for mime type and media type to make sure all bases are covered. Document any updates to the steps below.
3. Declare a code system fragment of the BCP13 code system of known media types (IANA + FHIR ExpressionLanguage  http://terminology.hl7.org/CodeSystem/v3-mediatypes  known expression languages (e.g., SNOMED ECL, OWL...)
4. Remove/retire the existing FHIR ExpressionLanguage code system: ttp://hl7.org/fhir/codesystem-expression-language.html
5. Create a new version of the existing value set http://hl7.org/fhir/valueset-expression-language.html, or a new value set to be based on all codes in the BCP13 code system fragment 
6. Note: by definition, any additional expression types used would be valid as long as they comply to the syntax defined in BCP13, however the community should be encouraged to register the additions with IANA.