The Roman Army of Diocletian

[Renatus]

I understand that, but I don't understand why that would make the Notitia Dignitatum a problem. They did not make the list, period. It's the units that we know should be there but aren't which make it problematic.

The problem would be if Steven believed that he could use the Notitia to calculate the size of the Diocletianic army without appreciating that some units, as you put it, "did not make the list" because they had been lost in battle. He is probably fully aware of that but that is the point that Adrian was making.