There is currently an extension defined on PlanDefinition.action.trigger   'ext-us-ph-namedEventType' 

This seems to have been added to support binding the trigger to a valueset instead of using the TriggerDefinition.name filed as that is a string value.  As the FHIR specification states that string elements can be bound to valuesets (see https://www.hl7.org/fhir/terminologies.html#strings) is there a need for this extension or can the TriggerDefinition.name field simply be bound to the valueset to achieve the same result without the need of the extension.