Aerarii

The aerarii were a class of Roman citizens not included in the thirty tribes of Servius Tullius, and subject to a poll-tax arbitrarily fixed by the censor. They were:

The inhabitants of conquered towns which had been deprived of local self-government, who possessed the jus conubii and jus commercii, but no political rights. Caere is said to have been the first example of this. Hence the expression "in tabulas Caeritum referre" came to mean " to degrade to the status of an aerarius."
Full citizens subjected to civil degradation as the result of following certain professions, of dishonourable acts in private life or of conviction for certain crimes.
Persons "branded" by the censor.

Those who were thus excluded from the tribes and centuries had no vote were incapable of filling Roman magistracies, and could not serve in the army. According to Theodore Mommsen, the aerarii were originally the non-assidui, excluded from the tribes, the comitia and the army. By a reform of the censor Appius Claudius Caecus in 312 BC these non-assidui were admitted into the tribes, and the aerarii as such disappeared. But in 304, Fabius Rullianus limited them to the four city tribes, and from that time the term meant a man degraded from a higher to a lower tribe, but not deprived of the right of voting or of serving in the army.
The expressions tribu movere and aerarium facere, regarded by Mommsen as identical in meaning, are explained by Greenidge the first as relegation from a higher to a lower tribe or total exclusion from the tribes, the second as exclusion from the centuries. Other views of the original aerarii are that they were: artisans and freedmen ; inhabitants of towns united with Rome by a hospitium publicum, who had become domiciled on Roman territory ; only a class of degraded citizens, including neither the cives sine suffragio nor the artisans ; identical with the capite censi of the Servian constitution.