Within the Functions tab, you must select the type of function you want to use, either System-Defined or User-Defined. System-Defined is the default.
Depending on which type of function is selected, one or two drop-down lists will appear below the radio group. If System-Defined is selected, the Function drop-down list will appear, allowing you to choose one of the predefined functions. If User-Defined is selected, two drop-down lists will appear, Core and Function. To select a function you’ve created, you must first select the core with which the function is associated. You can then select the desired user-defined function from the Function drop-down list.
Many of the system-defined functions convert a field’s storage type. For example, the CUR_STR function converts a currency control type field to a string control type field. The conversion doesn’t affect the way the field is stored in the table, only how its data is evaluated in the calculated or conditional expression.