A soggettario (in Italian) is simply a list of subject headings: it's a kind of thesaurus, used both as manual/tool for cataloguing and for browsing subjects and finding all books in a certain subject within the catalogue.
For instance the LCSH or the Nuovo soggettario. I can search a topic, browse the hierarchy and end up in the OPAC listing the titles.
So, does this whole thing have a single name in English? Do you use it?