http://hl7.org/fhir/StructureDefinition/elementdefinition-maxValueSet|5.2.0

This acts as an overall 'required' binding for an element that already has a less restrictive binding. It is used when there is a desire to have a 'small' enumerable binding that meets most needs and where the overall value set that the codes must be drawn from is 'infinite'. The the base less restrictive binding SHALL be a proper subset of the max binding valueset. In most cases, this extension is used where the base binding is 'extensible'. DEPRECATED: Use additionalBinding extension or element instead