A Compound Formula is an expression whose definition includes several Horizontal and/or Aggregate formulas.
Look at the following example. Given these transactions:
FlightInstanceAveragePrice = Sum(FlightInstancePrice) / Count(FlightInstanceDate)
FlightInstancePrice = FlightPrice if FlightInstanceNumberOfPassengers <= 100;
FlightPrice * 0.9 if FlightInstanceNumberOfPassengers > 100 and FlightInstanceNumberOfPassengers < 200;
FlightPrice * 0.8 otherwise;
FlightInstanceAveragePrice is a Compound Formula (defined in a global way).
The following image shows the "Flight" transaction being edited with the GeneXus transaction editor, and the FlightInstanceAveragePrice compound formula attribute being edited with the GeneXus formula editor:
Note that FlightInstancePrice is an horizontal formula.