Custom modules in MuleSoft provide the capability to create reusable components that can be shared across multiple Mule applications. Custom modules promote the best practices of modularization and abstraction, which are essential for building scalable and maintainable integration solutions. 

What Are Custom Functions?

In MuleSoft, DataWeave is a powerful expression language that is used for transforming data from one format to another. You can create custom functions in DataWeave to extend its functionality and make your transformations more modular and reusable. Custom functions in DataWeave can be defined using the `fun` keyword. Here is an example of how you can create custom functions in DataWeave: