A Centurion leads a Century and is generally considered a warrior without compare, these men usually have 10 plus years service under their belt and have proven beyond question their ability to lead.
A proven track record in command and strategy.
At least 5 years service and a proven record as a Decanus.
A Centurion is appointed by the leader of the Cohort, themselves the most senior Centurion in the Cohort. While not part of the formal process, all Centurions are expected to give their opinion on the appointment before hand. Generally a Centurion is considered the lowest rank of Officer and therefore a man of respect.
Management of the Century, administration of the district they are in, and generally keeping the peace in the Empire.