The Checkbox SDK is a .NET solution which contains source code for the front end Checkbox web application. This means that you get access to all of the source code for the web project of Checkbox. This does not include source code for back end code libraries, third party source code libraries, or any source code outside of the web project.
Having access to the web project source code allows you to extend and add to the Checkbox application. This allows you to add your own or external references to the code, customize existing references, add new UI pages, customize the UI, and create new custom item types.
For more detailed information on what the SDK contains vs the full source code please contact the Checkbox sales team at firstname.lastname@example.org