The core modules of Drupal PM are intentionally limited in scope. They exist to provide the foundation for an project management website or application and are meant to be extended by additional contributed modules. Contributed modules for Drupal PM provide features like invoicing, payment gateway integration helpers, reporting, alternate user interface, 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 / administrators. Developer documentation should be included in comments in the code and in .api.php files included in your modules.