Main military ranks of the imperial roman army

 

ROLE	RANK	ADDITIONAL INFO
Recruit	Tiro	-
Private	Miles	-
Engineers; gunners; administrative staff	Immunes	Soldiers chosen for their specific skills
Trumpeters and horn players	Cornicen, Buccinator e Tubicem	They transmitted commands with the sounds of their instruments
He carried the flag of the century, the cohort and the maniple	Vexilifer	It visually indicated the direction to follow to its reference structure
Bearer of the Legion's symbol	Signifer	He had to defend this symbol to the death
Bearer of the Eagle, symbol of the army	Aquilifer	He had to defend the Eagle to the death
Vice centurion	Optio	He was supposed to replace the centurion
Chief of a Century	Centurio	-
Highest centurion	Primus Pilus	Expert centurion
Head of the military camp	Praefectus castrorum	Experienced former Centurion
Direct subordinates of the deputy commander of the Legion	Tribuni angusticlavi	Citizens belonging to the order of Knights (rich people those were not members of the elite)
Deputy Commander of the Legion	Tribunus Laticlavius	Young senator
Commander of the Legion	Legatus legionis	Senator

Bibliography:

Storia completa dell'esercito romano di Adrian Goldsworthy

L'esercito di Roma di Chris McNab