This is from the "Perfect Guide" or something:
<<The Upper Domain and the Lower Domain>>
    
The given names "Upper" and "Lower" for the domains are nothing more than relative names. It is like how the third dimension is to the second, or how the fourth dimension is to the third. Each possess an individual time axis and space axis, but as a whole, they are elements that form one universe. In addition, it is unknown even whether or not the lower domain's concept of time exists in the upper domain in the first place. But regardless, although the existing role and function differ in each due to their difference in structure, the lower domain is indispensable for the upper, and the upper domain is necessary for the lower. This is because they form a single universe with both parts included. Therefore, the dysfunction and collapse of one will exert an effect on the other as well.
I guess is pretty clear about the different domains. Is just a single universe.