The identifying code for a particular manuscript used by its library. To cite a manuscript in full give the city, library, and full call number, then if applicable the relevant folios.
Manuscripts are usually alphabetized first by country, then city, then library, then call number.
Paying attention to how the manuscript is put together can be useful if you're trying to work out which bits of it came from where at what times and what might have been added or removed. Each mainscript number is a quire number. Each superscript number indicates how many folios there are in that quire. Colophons may be authorial or scribal. Indices of incipits and explicits can help with this sort of thing too.