Directory for content from external dependencies (base rule systems, additional modules) included as git submodules.
In a thriving open source ecosystem, you may want to include other works within your own - for example, base rule systems, settings, or optional modules on which your source book is based.
Use git submodules to add such works to the imports/ directory, and then include them from there, making sure to abide by the terms of their license.
This ensures that you refer to a specific revision of the work, that it is easy to find, and that output styles can be consistent.
Note that these external projects may set some variables which affect your own document following their inclusion (e.g. :imagesdir:).