In 2013, the UNG began exploring the galaxy for close-range colonisable planets. Their definition of close-range is within 100 light years of the Solar system. The majority of the UN colonies and the territory traditionally viewed to be within UNG control or jurisdiction is this 100 light-year area.
Long-range colonization fleets (specifically the new-Macross class fleets) are routinely sent beyond 100 light years to explore regions of space using Zentraedi star charts, in the hopes of finding colonisable planets. Due to the rarity of finding colonisable planets along their routes of exploration, as well as the sheer distance from Earth, any colonies in this area are generally considered to be outside of UNG control. However, due to the nature of their establishment, these colonies tend to be the most tightly bound to UNG jurisdiction; with some not being able to survive without continual input and supplies from the UNG.
The core of the UN has a diameter of 50 light years and is 50 light years 'tall.' Near the core of the UN is a 50 light year thick ring extending beyond the core. It too is 50 light years 'tall.'
Sectors 3 through 10 start 100 light years distant from the Solar system, and end approximately 200 light years from Earth. The area within the first 35 to 75 light years (135 to 175 light years from Earth) has been explored the most and has the largest number of colonies. They are all 50 light years 'tall.'
Sectors 11 to 26 begin at 200 light years from Earth. Their outer borders have yet to be placed, but will most likely be 300 to 400 light years from Earth. These sectors have only the most rudimentary of explorations to date, and will most likely not be explored and colonized en mass for another 25 to 50 years. The sectors are the wild frontiers of the UNG, and at least one sector is under the control of a non-UNG allied group.
Why 50 light years 'tall'? The Milky Way Galaxy is about 3,000 light years in thick, 80-100,000 light years in diameter, and about 250-300,000 thousand light years in circumference; in other words: expansive, but quite thin. The author has opted for the creative interpretation that in the Orion/Local Arm, the majority of the stars are within 75 light years from the Ecliptic (the orbit of Earth around Sol.)*
Above the 'prime' sectors of the UNG are the Roman numeral layer and below are the Greek letter layer. Thus, there is Sector 1, Sector 1I, Sector 1?. The thickness of the Roman and Greek sectors is significantly greater than 50 light years, however the number of star systems drops, and the distance between them increases considerably, the further one travels from the ecliptic.
As the UNG is concurrently searching for colonisable planets and Protoculture Ruins (or actual Protoculture beings,) it has decided to focus on the 'prime' layer due to the greater number of star systems; thus the greater chance of success. This is not to say that the Greek and Roman layers are being ignored, they too are being explored, but at a far lower rate.
*Note: not the Galactic_plane, which is actually the proper center-line of the Orion Arm.