We do not have user guide that explains everything. We write tutorials as our users ask for them when they stuck. We built control panel in intuitive way so that users do not ask much about how to use those options.
Regarding user guide for modifying code we do not have it as well. Because we wrote script for users who do not want or do not know how to code. Those who know some coding we suggest using NetBeans for PHP, there you can navigate through code with CTRL+mouse click on any model names and variables. Code is written in MVC format.
If you stuck in any place while coding we will try to help you and explain how it works. If you want to add feature which is not built in like adding custom table to database or adding integration with external APIs then you have to do it yourself. We still can suggest how to add database table and “model” class for it.