However, this article is mainly concerned with the meaning of "an office" (the modern word office derives from it) or "bureau" in the sense of a dignitary's staff of administrative and other collaborators, each of whom was called an officialis (hence the modern official).
The Notitia Dignitatum gives us uniquely detailed information, stemming from the very imperial chanceries, on the composition of the officia of many of the two Roman empires' leading court, provincial, military and some other officials circa AD 400. While the details vary somewhat according to rank, from West to East and/or in particular cases, in general the leading staff would be about as follows (the English descriptions and other modern "equivalents" are approximate):
Below those "dignities", there were often a few hundred minor officials, often slaves or freedmen, doing the clerical drudgery, not deemed worthy of any more detailed mention. They are only referred to collectively, by various terms in the plural, such as cohortalini (apparently the diminutive of cohortalis, see cohors amicorum).