Modules are one of the two ways to split a large single script file into multiple files.
The other way, include files, is designed for sharing code between multiple scripts. Modules, on the other hand, let you take what would be a single large file, and split it into multiple parts. When Opus runs your script it joins the files back together in memory, so in effect what you still have is a single large file - however you can work on it in separate parts, keeping related functionality together.
Module filenames must begin with the main script's name, followed by the module name, and have a .osm suffix.
For example, say you have a big script file called MyScript.js that you want to split up three files to make it easier to maintain - initialisation code, utility functions and the main script logic. Your files might be called:
Unlike include files, you don't need to explicitly say that you want to load a module - when the base script is loaded, all its modules are loaded as well (this is why the filenames are structured as they are - so that Opus can identify modules automatically).
The script editor lets you create new module files for an existing script.