One of the novelties of the last Android 4.4 (KitKat) version was the addition of the support to the secondary external storage, i.e. to the microSD card, allowing developers read and write only in the application-specific directory for security reasons, something that was very limited for both developers and users.
With Android 5.0 (Lollipop) This support has been enhanced to allow applications manage any directory of the microSD card but in a transparent and safe for the user, since it is the user that It decides that folder of your microSD card wants to access a given application.
With the new APIs and new intent ACTION OPEN DOCUMENT TREE applications may ask that our microSD directory we want to manage using the new selector, files and folders in Android 5.0. We can give permission to a specific folder with its respective subfolders or give permission to all the microSD by selecting its root folder. Once you select a folder applications may create, move, and modify files and folders within the selected directory tree.
In this way, from Android 5.0 users can manage completely your SD card safely through third-party applications, since thanks to the selector will always know that files can access each application.