Collected Editions: Often called "critical editions", these are complete, scholarly collections of works by a single composer. They may include critical commentary, significant historical context, and facsimiles of manuscripts to supplement the works contained in each volume. These are classed in 780.
Facsimiles: These are high-quality (and usually very expensive) scans of composers' original manuscripts. These are especially useful when researching the genesis of a particular piece or tracking the evolution of a composer. Facsimiles are generally classed in 780, but you can quickly find them with a "facsimiles" subject search.
Monuments: These are scholarly anthologies of works that fall under a broad theme or subject, usually by geographical region and/or type of music. These are classed in 780.82.
Siroe, re di Persia : opera in tre atti, HWV 24 by 
Le nozze di Figaro : K. 492 : facsimile of the autograph score, Staatsbibliothek zu Berlin--Preussischer Kulturbesitz, Biblioteka Jagiellońska Kraków (Mus. ms. autogr. W.A. Mozart 492), Stanford University Library, the Juilliard School Library by 
Wiener Instrumentalmusik vor und um 1750 : Vorläufer der Wiener Klassiker [...]