Creating a new Environment from an existing/old Deployment can be very beneficial. Let's look at a simple example. You have a bug in your Production Environment that you need to fix. Your Staging Environment is already a couple of versions ahead of your Production Environment and you don't want to block the teams that are working on the next production release. You can thus just clone the Deployment in your Staging Environment that was used for the last Deployment to Production into a new Environment and test your hotfix there before updating Production.
You can create a new Environment from an existing Deployment in any other Environment from the App Details Screen.
Click on the three dots next to the Deployment you want to clone into a new Environment.
Select the Environment you want to clone this deployment set to or select + Create new environment.
If you selected + Create new environment:
Environment type - allows you to select the Environment Type that should be used for the new Environment.
Environment name - specific the name for the new Environment following your internal standards.
Click on Create.