Among the most important sources for the study of the Roman military are “military diplomas.” The Roman army was comprised of both citizen and non-citizen soldiers – the formerly mostly in legions, the latter mostly in auxiliary units and fleets. When non-citizens completed their service they were granted Roman citizenship; until 140 CE, their families were granted citizenship as well. One copy of a diplomas was given to the soldier, the other one kept in an archive in Rome.
These citizenship grants are recorded on bronze military diplomas, which were issued en masse to retiring auxiliaries in a given province. Though the content of such diplomas is extremely formulaic, they nevertheless provide valuable information. First, because they were issued en masse, they indicate which military units garrisoned in a specific province on a specific date. Second, despite their formulaic nature, they give some indication of the soldiers comprising these units: we can, for instance, ascertain the ethnic composition of one or another unit based on onomastics. Third, we can get a sense of the movement for soldiers after retirement based on the place where the diploma was discovered. Fourth, they provide valuable information about the commanders of these military units, and we are able to ascertain much about the career of high level commissioned officers and provincial governors as a result.
Terminologically, scholars distinguish between the diplomas issued to individual soldiers and the constitution issued en masse to all new-citizens soldiers on a given day. All diplomas on a given day contain almost exactly the same wording and information with the exception of the soldier’s name (and his family members’ names), unit, and commanding officers.
Despite the sturdy nature of Roman bronze (as opposed to, say, papyrus), most diplomas have been lost to time – ultimately melted down an reused for other purposes, such as coinage, monuments, or other goods.
Andreas Pangerl and Werner Eck have produced a very informative website for more about diplomas, including extensive photographs and more thorough information on the matter. Werner Eck, undoubtedly the premier expert in the matter, also regularly updates his Academia.edu profile with his latest publications, should one wish to stay up to date.