Plugins, Custom Web Publishing, and Custom Functions are how capabilities are added to Filemkaer.
Some are web based (Custom Web Publishing) (see http://fmwebschool.com)
Some are IWP compatible, most are not.
Custom Functions - see https://www.google.com/search?q=custom+functions+site%3Afilemaker.comAbout custom functions (FileMaker Pro Advanced)The Custom Functions utility lets you create customthat can be reused anywhere in the database file in which they are created. You can copy and paste custom functions between FileMaker Pro files or you can import custom functions from other FileMaker Pro files. Once are written for the function, they do not have to be rewritten to be applied to other or used in other scripts.You can maintain and edit custom functions and the formulas they contain in one central location. Any change made to a custom function will be copied to all instances where that custom function has been used.Custom functions can be used:•anywhere in a file•by any user or only by users with full access privileges•across multiple platforms•to replace third-partyfor some purposesCustom functions will run in both FileMaker Pro and FileMaker Pro Advanced. Newly defined functions appear under their own category in the functions list of the Specify Calculation and Edit Custom Function dialog boxes. If you do not wish to reveal your custom functions, you can disable their display in these dialog boxes.Note FileMaker Pro Advanced may not be available in all of the shipping languages of FileMaker Pro.