MODULAR CODE is portable code: Chunks of code that can be easily easily moved from one solution to another with minimal modification. Each chunk of code addresses a specific, discrete need–account management, error handling, etc. FileMaker's structure is so dependent on context that modular code is difficult to write.
Darren Burgess of Mighty Data discusses the ins and outs of creating modular code and explains how the community helps improve the code with feedback. Check out his article if you are interested in writing modular code.