Modules.php
Modules.php is located in your ra-content directory and is included in every Review App script to give you access to your custom functions on any page. By default this script loads any properly-configured plug-ins.
You can add your functions to supplement, build on, or replace certain built-in Review App functions. For example, you can create a function in modules.php called ra_post_submit() that is executed after a user submits a form on your site to do any custom data processing. Similar functions are available for new user registrations and user password updates (helpful for integrating multiple software logins).
See this article for a partial list of available Review App functions you can built on.