His responsibilities involved monitoring the state's revenues and expenditures and maintaining the accounts of the fiscus, giving the a rationibus considerable influence.
[1] The role of the a rationibus was originally created by Augustus, who needed accurate and comprehensive accounts of the state's finances in order to exercise budgetary control, and was thus given to members of his household, probably freedmen.
[5] Sometimes, the offices of the a rationibus and ab epistulis, the secretary in charge of the imperial correspondence,[6] were joined, e.g. in the case of Tiberius Claudius Vibianus Tertullus.
However, from the 2nd century AD on (i.e., around the reigns of Trajan and Hadrian), the position was occupied only by Equestrians (Equites) after the reputation of freedmen had been blackened due to their undue influence at the imperial court and several corruption affairs.
The office of the a rationibus was abolished through Diocletian's tetrarchic reforms, which put the management of the imperial finances during the 4th and 5th century AD under the purview of the comes sacrarum largitionum (master of the sacred largess).