Segmentum

The space ruled by the Imperium is divided into four major territories known as Segmentae Majoris, known simply as segmentum.

Overview
Each segmentum is administered by a Marshall who oversees all of the administrative and military processes in their sector. They are responsible for collecting the Imperial Tithe, as well as ensuring that the many worlds that belong in their segmentum remain loyal to Terra. Marshalls are able to summon forth their segmentum's vast fleets, but theoretically speaking, they would not be able to fully command them as their movements could only be dictated by the segmentum's respective Grand Admiral.

Ultimately, all of the marshalls are subservient to the High Marshall.

Tips
Hi, it's the author here. So as you can see above there are a bunch of sectors and segmentae to consider, but u dont hv to worry abt them too much. u can learn more abt each and one of the planets that belong to one of these sectors but it wont affect u all that much.

its mostly just me bein a loregeek lmao