A collection is a group of proofs that have been created at the same time. You will know when a proof belongs in a collection if you see this icon on the proof screen:
Managing your proofs in a collection
If you are an owner of one or more proofs in the collection, after clicking the collection icon, you will see a cog icon. Click the cog icon to reveal the manage collection pane where you can perform these actions:
Edit the collection name
Add an owner to all the proofs you own in the collection
Add a reviewer to all the proofs you own in the collection
Replace the approver on all the proofs you own in the collection
Archive/reopen a collection folder
If you add an owner or add a reviewer to the proofs you own in the collection, a single email notification will be sent. This is a great way to perform bulk actions on a group of proofs you own.
Note: You can always add new proofs to a collection by clicking the collection folder you want to add to, then using the file uploader to drag & drop a proof/s.
If you want to know how to remove a proof from a collection, check this article for more information.