The Spaces button (Short for Workspaces) allows you to filter by the Space a Fax has been assigned.
By default, a phone number receiving a fax will route items to the Group Workspace. You can change this in the Menu > Admin > Fax Settings. You can either select Group Workspace (routing items to the Practice Space) or you can route to an individual user (Private Space)
For more information, please refer to Updox's documentation.