Digital Asser Library is the place where you can store your approved designs for future re-use. In Approval Studio, you can access DAL by choosing it in the right upper view panel. Your next step will be checking the box in the left panel to enable the Digital Asset Library. Now it's settled.
To add an asset to your DAL, go to the Project - Files where your asset is. Then click on the three dots for the drop-down menu, where you click Add to DAL and choose a folder if you have any.
Once done, you can add Description and Tags to the now DAL asset, and see its History. By clicking the Copy to Project button you will open a menu that will let you choose the project from the existing list and edit the assest's name before pasting it if need be. Finally, you can always Delete the asset.