I looked at a map of Roman Provinces and Diocese after Diocletian's reforms and I was just wondering why the provinces and diocese had the shapes that they did. While some seem to follow rivers, others seem kind of arbitrary, so I was just wondering if anyone has an answer, like is it based on population or something else.
Diocletian largely went about cutting up the preexisting territories in order to improve management and tax collection. Some of those lines probably date back to the extent of territorial gains when they were formed or perhaps the indigenous boundaries recognized by the people of those lands.
I might think that territories could run along geographic features not readily found on a map or perhaps even minor roads.
Hoped this helps