What that was about was that all gui apps on riscos only ran one process, no matter how many files you had open. These machines had very little memory, so managing it was very important - there was actually a system panel you could open (I forget it's name) where you could drag sliders to change how much various things were allowed to allocate.

The downside, of course, was that if some app crashed, it would take out every file you had open with it. But then, it didn't really have very good isolation, so often a crashing app would take down the whole OS.