The Red and the Black
The Third Imperium is over 1000 years old, and encompasses a massive swath of charted space, including over 11,000 worlds. The Second Survey was completed by the Imperial Interstellar Scout Service (IISS) in 1065, taking 70 years to compile. The empire is based out of the Core Sector, and has been ruled by a succession of emperors and noble families, as well as a large and mostly stable bureaucracy. It is not the only interstellar government of notable size, although it is the largest in the known space.
News and orders travel slowly. By X-Boat it takes many months or even a year or more for information to travel across the Imperium, meaning that while there are standing orders, norms, and expectations for imperial forces and governments, there is also a great deal of local autonomy. The development of an entrenched aristocracy over the centuries has contributed to stability and consistency across space, but has also given rise now and again to strife between families and factions seeking to maneuver for power.
In reality, the Imperium is better described as a semi-feudal confederacy, with affairs on most worlds being the concern of local rulers and only those on important worlds and concerning trade and the space lanes being under the influence or control of the Imperium.