This is a cloud storage service with unlimited capacity in your Dropbox account, available when you use the Professional plan.
You can use unlimited cloud storage by joining the Dropbox Business team (Professional DJ team) using your Dropbox personal account.
When you use Cloud Library Sync, you can upload and save unlimited amounts of music and video files to the cloud.
You can keep using files that are already saved in the cloud in your Dropbox personal account by transferring the files to the Professional DJ team.
Find out more details here.