A shelf is populated by a rule. The predicate is shown under every shelf name and in the inspector. Descending the tree narrows the query — a child's rule ANDs with its parent's.
People augment it, and it shows. Rule-matched is the unmarked default; hand-added carries an added marker with who and when; excluded is struck through and only visible to a curator.
Permissions rule the day. Switch View as in the top bar and watch the same org shelf change contents. The definition is shared; what you see is yours. Every count is post-filter — never a total.
One library, one lens. Catalog asks what kind of thing this is; Workspace asks where the work is happening. A library never mixes them.
Fresh vs unread are different. Fresh is a property of the object and can be part of a rule. Unread is a property of you-and-the-object, so it is only ever a filter.