A Catalog is a list* of item definitions that are used by a Game. Anything of which a character can have multiple items - something that fits one of the Collections - is an entry from a Catalog.
Catalogs are seen differently by Players and Storytellers. Players see a collated version of all the Catalogs of a type that have been included in a Tabletop; the Storyteller sees the individual Catalogs and their sources. Regardless of the source, the entry and creation of a Catalog are the same.
(*Actually a dictionary)