Officium (pl.: officia) is a Latin word with various meanings in ancient Rome, including "service", "(sense of) duty", "courtesy", "ceremony" and the like. It commonly also referred to the office of a magistrate and his sometimes numerous staff, 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 leading court, provincial, military and certain other officials of the two Roman empires c. 400 AD. While the details vary somewhat according to rank, from West (Rome) to East (Byzantium) 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 several hundred subordinate staff, sometimes slaves or freedmen, performing day-to-day administrative duties, 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 diminutiveofcohortalis, the term suggesting a large number; see cohors amicorum).