The Notification Center regroups all the notifications regarding audio file exports, spreadsheet imports and Spotify imports.
When one of these actions takes place, their progress and completion are displayed in the Notification Center.
You can click on any of the completed tasks to access its corresponding resource.
For instance, if you imported an album through the Spotify Importer, you can click on its notification to open up that album.
Here is a video showing you where the Notification is located and how you can open up an album through its pop-up:
The Notification Center will also allow you to know when your spreadsheets have been imported or when a contact you sent a playlist to has opened it, etc.
It is also through the Notification Center that you can download metadata or audio file exports.