SQL server database backup - Destination Disk - Adding multiple files - does it duplicate or split backup into the files?

When we do full data backup (using SSMS UI), at the bottom of the window we have the option to specify the destination as Disk and also to add multiple files. My question is - does adding multiple files create duplicate copies of the full backup? or does it create a split backup - that is split the full backup into the specified files? This book suggests it does a duplication where as this link suggests that it does a split. Please can someone clarify.