The OTE is a full clone of the production software designed to be as close to the production environment as possible to facilitate effective pre-production testing. There are however a few important differences which are noted in the Differences from Production Environment page.
All domain registrations and other purchases are made against a test environment which is not live. This provides you with a safe environment to test your integrations and run unit tests before exposing your software to your own customers.
It is strongly recommended that comprehensive testing is performed on the OTE prior to production use of the API as this will help to eliminate integration issues.