http://hl7.org/fhir/StructureDefinition/parameters-definition|5.2.0

This specifies the definition for a parameter if needed. This is useful for communicating the type of a parameter when the parameter has no value.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/ParameterDefinition