You can create any number of subfolders in a personal folder. To create subfolders in a shared or public folder, you need to have the required permissions. Depending on the app, you can create folders or subfolders.
In the Email app you can create folders and subfolders.
In the Files app you can only create subfolders, not folders.
In the Address Book, Calendar, Tasks apps you can only create folders, not subfolders.
How to create a new folder:
Depending on the app, the procedure differs:
In the Email app click on below the folder tree.
In the Address Book, Calendar, Tasks apps click on or on in the folder tree.
Note: In the Files app you cannot create folders, only subfolders.
Enter a name in the New folder window. Click on .
How to create a new subfolder:
In the Email or Files app's folder tree select the folder in which you want to create a new subfolder.
Note: Select a folder for which you have the permission to create subfolders.
Enter a name in the New folder window. Click on .