Custom functions are without a doubt one of the most powerful features in FileMaker; we cannot advocate their use strongly enough.
When added to a file by a developer, custom functions become available in various calculation dialogs as additional functions for use within expressions. They are snippets of code that, just as FileMaker’s preestablished functions do, accept parameters and produce output. One example might be
fnCommission ( unitPrice; quantity; discount )
This function would presumably return a dollar amount based on some formula that multiplied
quantity, subtracted a
discount from the total, and then applied a percentage or some internal factoring to arrive at a sales commission.