When you submit your app, you should provide instructions on how to test your app during your app review.
Including instructions on how your app should be used lets us give you valuable feedback if they encounter issues while testing. If you have an app that offers a lot of functionality, then you should consider including a link to a video walkthrough of your app. This helps us understand how to use your app.
If your app connects to other services, then you must include test accounts. For example, if your app requires account access to a marketplace, then you must include credentials for a test account on that marketplace. If testing your app requires a test account and you don't provide one, then your app submission will be rejected.