Short answers, pulled from the story.
Emperor Diocletian established the Roman diocese system in the year 296. This reform multiplied provinces from forty-eight to around one hundred and created twelve initial dioceses covering the empire.
Each Roman diocese was governed by an agens vices praefectorum praetorio or simply Vicar under the Praetorian prefect. These vicars controlled provincial governors and heard appeals decided at the provincial level.
Constantine I transformed the ministerial Praetorian Prefect into a regional Prefect creating specific territories called Praetorian Prefectures containing several dioceses. This administrative decentralization reduced the utility of the dioceses themselves and caused the power of vicars to decline.
In 535 and 536 Emperor Justinian abolished the dioceses of East Asia and Pontus while also splitting the diocese of Egypt into five independent circumscriptions. He aimed to simplify bureaucracy and decrease state expenses since their courts of appeal were used less frequently.
The Roman diocese system disappeared in the seventh century following the establishment of first themes which held both military and civilian authority. The last certain attestation of the Praetorian Prefect of East is in 629 while most Balkans fell under Slavs before the end of that century.