You need Advanced to store custom functions. However you can still use whatever function you like on a per field basis.
FileMaker Advanced is required to add or import a custom function into a FileMaker File. Once that is done, the function can be used throughout the file even if it is opened with FileMaker Pro.
What Rick is saying (as I understand it) Is that if you don't have Advanced and find a custom function that you like, you can often adapt that custom function calculation to use it in a single calculation field.