Shelf allows you to connect different cloud accounts - and maintain that connection - at the same time. This way, if you have important content in different locations (e.g. certain important files in Google Drive, other important files in Dropbox) you can search them, view them, and access them in a single location. On the Import Cloud Accounts page in Shelf there is a section which list your current connected cloud accounts and by selecting the drop-down arrow to the left of each account you can view which files are synced from that specific account.

At this point in time, Shelf only integrates with Google Drive, Dropbox, and One-drive. You can sync one cloud account to a single Shelf account but you can perform content imports from multiple cloud accounts.