It is a development plan for you to setup your site before launch. It allowed up 50 to orders. It doesn't allow to add your own custom domain.
We will be remain the store owner during the development period in order to keep it in free development plan. When you site is ready, we will transfer the ownership to you, you will have the full access and the ownership of the account. Then you can pick one of the shopify plan to remove the limit.
