Sandbox is an easy way to test your integration. When you initialize a sandbox session, a warning will be displayed in the Verify UI.
It means that everything sent through this session will not be processed. It will only be for integration testing purpose.
Once you upload something, a task will be created to your manager and you will be able to chose the outcome you want to simulate. Webhooks will be sent as if it was in production. It is a good way to test your notification system too.
You can access to your Sandbox tasks in Developer > Sandbox Tasks. You can then simulate the outcome you want by clicking to the associated button. You can create as many sandbox sessions as you want.