Based on ballot coments, particularly Lloyd Mc., VSD has two type of Content Logical Definition (CLD) types: Computable and non-computable. The former is basically ValueSet.compose. The latter intends to support a textual decription of the definitional representation of an expression that could be followed to determine the value set expansion.

VSD expalins this thusly: Some descriptions of Code System content cannot be captured in a computable syntax. This can occur for a variety of reasons, such as when the needed codes are not identified using computable parameters (e.g., "the codes representing conditions currently under discussion in the US Public Health blog") or when an initial version of the CLD is crafted as a general textual statement using "pseudo-code". In this case the CLD can use text to capture the CLD mechanics. As noted before, if a CLD uses this approach, it cannot be combined with a formal computable syntax CLD, i.e., that version of the Value Set Definition will not support a normative directly-computable Value Set Expansion Code Set. Of course, a different version of the Value Set Definition could specify a CLD that uses a computable syntax.

There is no support for this in FHIR valueSet. I suggest that we consider using the same solution as I propose in 18135 wherein we could use Expression but need to also support use of simple text as a value for Expression.language (so add text/plain to ExpressionLanguage code system.)