A. = Archivio, Archive B. = Biblioteca, Library M. = Museo, Museum -> = This is an old sigla used in some Rism series; prefer the new one
The code appearing with some siglas is a link to the data given by the Anagrafe delle biblioteche italiane where you can find more details: opening hours, services, collections, etc.
For Church libraries see also Repertorio delle biblioteche ecclesiastiche italiane by ABEI, Associazione delle biblioteche ecclesiastiche italiane.
Among the notes, Cabimus shows the card number in the Guida alla biblioteche e agli archivi musicali italiani, Siusa links to the web page of the Sistema informativo unificato per le Soprintendenze archivistiche, managed by the Direzione Generale Archivi.