Sources about the praefectus castrorum

[Nathan Ross]

I have already found a list of sources about different camp prefects but I cannot understand what is the meaning of the abbreviations

The place you took the list from ought to have a key to the abbreviations they've used - check at the beginning, maybe?

PIR (volume number) is Prosopographia Imperii Romani, a German-language digest of individuals mentioned in sources and inscriptions. It's online here:

Prosopographia Imperii Romani

CIL is Corpus Inscriptionum Latinarum, another collection of inscriptions; AE is L'Année épigraphique, a French version of much the same thing (confusingly). These and most other epigraphic collections are online in the Clauss/Slaby database:

Epigraphik Datenbank

Type the reference into 'Publication' and you should get the text, but it's a bit temperamental: only use arabic numerals, not Roman ones, and put a comma after the main number...