First Referenced:
'I was told that everyone I'd served with in the military died shortly after I left. (Part 5)'
The Branch Worlds are parallel realities, which appear to have been derived from and grown by the Brownshirts while assisting The First World in building the fortifications of the Inner Shield and Outer Shield. These realities serve to disperse forces exerted on the two shields and provide structural support, similar to the supporting structures within any typical building.
Little is known about the Branch Worlds, other than that a number of them also support the vast machinery required to generate and maintain the two shields.