There are a few value set bindings where restricting the allowed members to be used in a coded element needs to be restrcimted to a known upper limit of values. This is done using MAX binding as described in Core Principles. How and under what restrctions this can be done in FHIR does not seem to exist. This should be in the "Using Codes in Resources" page.

Of note, there seems to be two types of MAX, one where the max can be linked to a finite known set of values, such as This is not the same as country codes - ISO 3166 - because a finite set of codes is known therefore a max binding to this *is computable. *Others do exists when the max set may not be finite such as when we want users to pick from a code system defined by a grammar. UCUM, Human Language BCP 47, and perhaps MediaType which might be guided by BCP13.