Query catalogs display a filtered list of items. The filter is specified as a JSL script that you can enter as one of the catalog's properties.
For example this query catalog returns all Business Process Diagrams in the library (if you add a new Business Process Diagram to the library, a reference to it dynamically adds to the Query Catalog as well)
Note: As the catalog's members (the items listed in the query catalog) are determined by the script, you cannot remove items from a query catalog. You may however delete items from the Library via a query catalog.