Probably a custom function. Custom functions can only be added, changed deleted with FileMaker Pro Advanced but once added to a file, work like any other function. You probably can find this in the list of functions in Specify Calculation dialogs but not in FileMaker help as it doesn't come with the FileMaker application, it's added to the file.
If you have FileMaker Advanced, select Manage | Custom Functions from the File Menu and you should find it listed there.
That's what I thought it might be. I don't have Advanced. Doing things like that make it hard to learn how to use/modify the starter solutions. I'll just try to figure out what was done and a way around it.