The application supplies a context and processes the function using a mode of operation.
There are three modes of operation:
Functional mode
Event mode
Functional and Event mode
If you choose Functional Mode or Functional and Event Mode, a top expression has to be assigned to the function.
In the Event mode, you do not need to assign a top expression to the function.
For more information, see Modes of Operation
Creating a Function
Assigning Context and Result
Assigning Top Expression