What was the fundamental principle of Diocletian’s reformation of the Roman Empire?
There are at least two things that can be identified as fundamental principles of Diocletian's reforms. One of these is division of labor and the other might be called "divine right of kings."
Diocletian changed the way in which the Roman emperor was perceived. The emperor had once been seen as the person chosen by the people to lead them. Now, he was seen as the person picked by the godes. This is why he was now addressed as "dominus" (master) rather than "princeps" (first citizen).
Diocletian also split the empire in many ways. Most famously, he divided it into four parts and had it ruled by a tetrarchy. He also divided smaller parts of the empire, making (for example) more provinces that would be ruled by more governors. The fundamental principle there seems to be a division of labor to make for more efficient administration and to reduce the power of the various governors and officials.