How exactly does world composition work?

I still don’t quite understand how this all works. I used level splitter to split about 8k map.This creates a split level template and 9 sublevels and maps in a folder, Then enable world composition. When the split template is loaded it has like 9 subdivided squares , but the map is just black. Only when i load my regular map does the landscape appear again lit and normal. But then there is only one persistant level. So how do i work with this ? after the level or landscape split, Am i supposed to then load the group of levels or the split level template and work build my level from there? or would i again load my regular map and continue building and saving there? Or is it such that if i have the 9 level sections , i can only select one (or load that one) and build and save on that part of the map. I mean after the split , is my normal map not used at all anymore?(that is , the map i originally split?)

Does anyone know about this?