Neither purpose, useContext or any other existing FHIR element represents the idea represented in VSD as "Scope". This is a critical and definitional element in a value set as it describes the intended scope of concepts that should be included (or additionally excluded) from membership in the expansion set. This will always be intended for a human to interpret. I think many folks use ValueSet.definition for this but the current definition for the element is targeting a slightly different focus. In particular the phrase "The description is not intended to describe the semantics of the Value Set - there are no intrinsic semantics separate from the codes contained in its expansion." in the comments seems to preclude actually describing the intended scope of concepts.

The VSD definition for scope is: textual description of the span of meanings for concepts to be included within the Value Set Expansion Code Set including the intended use and limitations of the Value Set.

One potential resolution to this issue is in part to change the definition of ValueSet.definition to include the text above. Another potential resolution is to add a new element for scope, but I am less included to that resolution.