What is a module in context of Mill?

[justf0rfun]

Let's imagine we write an application. We try to avoid a monolithic structure and spaghetti code. Hence we split the application into parts, each responsible for a business or technical domain like customer management, car pool management, car leasing contracting, data storage, user interface etc..

That's what I would call modules. To avoid mixing up code and responsibilities we put the code for each module into a separate namespace and a corresponding file system directory. The application code would be mainly to connect all the modules together as well as bootstrapping and shutting down the program.

We go even further and put each of these namespaces into a separate projects and a corresponding directory. That would make the separation more strict and we can now reuse the modules in other applications.

What actually is a module in context of building artifacts from code with a build tool in general and Mill in special? How do I have to think about it?