The core modules of Drupal Commerce are intentionally limited in scope. They exist to provide the foundation for an e-commerce website or application and are meant to be extended by additional contributed modules. Contributed modules for Drupal Commerce provide features like coupons, shipping, payment gateway integration, and others.
This section of the handbook is devoted to contributed modules hosted here on drupal.org. Any contributed module can be represented by a child page in this handbook, and the documentation should target site builders / store administrators. Developer documentation should be included in comments in the code and in .api.php files included in your modules.