Unassigned Items
The Collection field is mandatory to create an Item in a Shared Vault.
However, an Item can be removed from a Collection after it is created.
As for Moving an Item Between Collections, open the Collections Screen for an Item.
Click Unselect all.
Click Save. The Item is now assigned to no collections.
Click the Unassigned link in the Filters Menu. The Item is displayed in Unassigned.