It might be just storage as it was brought up as an example of how subsystems of mainframes are essentially their own world and a single disk or distributed vol

It might be just storage as it was brought up as an example of how subsystems of mainframes are essentially their own world and a single disk or distributed volumes over a long distance are presented the same to the rest of the system. In the Linux world DRBD does something similar (just much simpler). The point, however, is that the software knows nothing about this being distributed.