In the Test stores section you can enter a list of stores that can see and use the integration before it goes live for all merchants. This can be helpful for development and beta testing phases, or if you want to have a private integration. Enter URLs with the myshopify.com domain, and separate them with a comma without any spaces. For example, teststore1.myshopify.com,teststore2.myshopify.com.
Next up is the app setup overview