http://hl7.org/fhir/StructureDefinition/capabilitystatement-search-parameter-combination|4.0.1

This extension defines a possible search parameter combination, by listing a set of search parameters and indicating whether they are required or optional. If a search combination is specified, clients should expect that they must submit a search that meets one of the required combinations or the search will be unsuccessful. If multiple search parameter combinations are specified, a client may pick between them, and supply the minimal required parameters for any of the combinations.