A trigger-match links an actual service (e.g., an observation or procedure that took place) with a service in criterion mood. For example if the trigger is "observation of pain" and pain is actually observed, and if that pain-observation caused the trigger to fire, that pain-observation can be linked with the trigger.
\nhttp://terminology.hl7.org/CodeSystem/v3-ActRelationshipType
where concept is-a MTCH