Custom functions are without a doubt one of the most powerful features in FileMaker; we cannot advocate their use strongly enough.
Simply described, custom functions become available in the calculation dialog as additional functions for use within expressions. They are snippets of code that, just as FileMaker’s pre-established functions do, accept parameters and produce output. One example might be
fnCommission (unitPrice; quantity; discount)
This function would then 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 as a sales commission.
Note that as a developer ...