The Empire (and, likely, the Republic before) of Nilfgaard is divided into several administrative units so that its territory can be effectively governed. They can, but don't have to, reflect the pre-conquest borders. The degree of Imperial rule varies, from strictly controlled provinces like Geso to lands where Imperial Governors and Palatines formally as "advisors" to local puppet monarchs (Maecht, Lyria and Rivia).
Usually the largest unit, governed by a Governor. It is divided into sub-units known to Nordlings as "marches", though this word doesn't denote the status of given land. For example, the unit Province of Metinna consists of "march" Metinna and "march" of Maecht from the administrative point of view, but both Metinna and Maecht are formally kingdoms with their kings and royal courts.
The Empire consists of more than a dozen provinces.
Usually smaller than a Province, governed by a Palatine.
(Note: is a Palatinate just an official name for aforementioned "march"?)
The smallest unit, usually being a city with adjacent terrains. It is governed by a Prefect.