For an outpatient measure we discussed using the Encounter, Performed class attribute to find the right types of Encounters. So we wrote code as

["Encounter, Performed": class in "Outpatient Activity Codes"] Encounters
  where Encounters.relevantPeriod overlaps "Measurement Period"
    and AgeInYearsAt(start of "Measurement Period")>= 18

However, when importing this measure into Bonnie when you go to create a test patient there is no option to add an Encounter, Performed. It seems Bonnie ignores the Encounter, Performed since it is not bound to a valueset on code