What are sub-modules in ES6?

Suppose I have a ES6 module named a.js. a.js modules imports b.js and c.js modules. Similarly b.js imports d.js and c.js imports e.js.

Now what I want to know is which ones are sub-modules and super-modules.

I feel b.js and c.js are sub-modules of a.js. But I am not sure. Really confused. Please help.

Thanks in advance.