http://hl7.org/fhir/StructureDefinition/valueset-expand-group|4.0.1

This extension declares a group of concepts that is generated into the ValueSet.expansion.contains hierarchy when the expansion is generated for a UI. THere is no inherent assigned meaning to the hierarchy; it is used to help the user navigate the concepts. Each group has a display and/or a code, and a list of members, which are either concepts in the value set, or other groups (by code).