Source vs. import files, when a file is considered source and when it is considered import?

help
1

It is not clear on the doc:

When a less file is considered source and when it is considered import ?

When a sass file is considered source and when it is considered import ?

2

There are two different types of files recognized by this less package:

Less sources (all *.less files that are not imports)
Less imports:
files with the import.less extension: .import.less
files in an imports directory: /imports//.less
marked as isImport: true in the package’s package.js file: api.addFiles(‘x.less’, ‘client’, {isImport: true})