Assigning Top Expression 
You need to assign an expression as Top Expression when you choose Functional or Functional and Event mode of operation.
For more information, see Modes of Operation
Open the function in the Object Manager panel.
Assign an existing expression or a new expression as the top expression.
Assign an existing Expression
In the Detail section, under Properties tab page, choose the mode of operation.
Choose
(Graphical Access) next to the Top Expression field.
In the context menu, choose Select....
In the Object Query dialog box that appears, select the expression from the list.
Create an Expression
Choose
(Graphical Access) next to the Top Expression field.
In the context menu, choose Create....
In the Object Creation dialog box that appears, enter the following data in the General Data section:
Name
Short Text
Select the type of expression from the Type field and choose Create or Create and navigate to object.
Note
If you choose Create the newly created expression appears next to the Top Expression field.
Creating a BRMS Connector Expression
Creating a Constant Expression
Creating a DB Lookup Expression
Creating a Decision Table Expression
Creating a Decision Tree Expression
Creating a Function Call Expression
Creating a Random Number Expression
Creating a Value Range Expression
Creating a Search Tree Expression
Creating a Static Method Call Expression