"The Making of the Roman Army: from Republic to Empire" by Lawrence Keppie.
The "badge of rank" for a Centurion was a vinestick (vitis), similar to the baton carried by some British and American officers. The vinestick did not only denote rank: it was also employed as an instrument of immediate discipline.
Other elements in the uniform of a Centurion that distinguished him from an ordinary Miles were: wearing his sword hanging to his left side, rather than to his right; the crest on his helmet sideways, instead of fore-and-aft; silvered armor; and greaves protecting his legs.
Most Centurions also wore decorations and awards garnered in their service. The Cenotaph of Marcus Caelius, a Centurion of XVIII Legion, now in the Rheinisches Landesmuseum, Bonn, Germany, shows him wearing an oak-leaved corona civica (for saving the life of a fellow citizen) on his head, torcs suspended from shoulder straps, four medallions (phalerae) on a harness (other medallions are hidden behind his arm), and bracelets on each wrist. The awards are appropriate to a centurion. Caelius was MIA at the battle of Teutoberg Forest.
The legion's Aquilifer was also invariably a trusted and highly decorated soldier. The gravestone of Cnaeus Musius, Aquilifer of XIIII Gemina, who died at some time before AD 43, is preserved in Mittelrheinishes Museum, Mainz, Germany. Over his mail shirt he is wearing a harness supporting 9 medallions; at his shoulders are two torcs, and on his wrists two bangles. Strangely, perhaps, it seems that Aquilifers wore no helmet. This contrasts with lesser standard bearers (e.g. Signifers) who wore animal skin hoods.
More senior officers, such as the Military Tribunes, wore elaborate crested helmets and distinctive clothing, such as molded cuirasses (6-pack muscles, etc.). END QUOTE
Here are the ranks, in descending order, for a Legion during the classical period, meaning the period when the legions formed the primary fighting arm of the Roman army.
Commanding Officer of the Legion: "Legatus Legionis" ( = Legate in English).
2nd in command of the Legion: "Tribunus Laticlavius" ( = Tribune). The "Laticlavius" extension of the rank's title refers to the broad stripe on this man's toga, which signified that he was of the Senatorial class.
3rd in command of the Legion: "Praefectus Castrorum" ( = Prefect, literally the Prefect of the Camp). Unlike the Legate and the Tribune, the Prefect was almost always a veteran soldier who had won promotion through the ranks of the Centurions.
Next, we have 5 officers of equal rank, the "Tribuni Angusticlavii", each of whom nominally supervised two Cohorts in action, but were actually more often engaged in administrative duties. The "Tribuni Angusticlavii" were supposed to be members of the members of the Equestrian Order (the Knightly class immediately below those of Senatorial rank in the Roman hierarchy); but frequently these "Military Tribunes" (not to be confused with the "Tribunus Laticlavius") were career veteran soldiers, not greenhorns appointed because of family connections.
The Centurions. These men were the true backbone of the Roman legion. Each legion had 59 or 60 Centurions, most of whom were responsible for the (typically, or at least at "full" strength) 80 men of each "Century". The Romans thought of them as NCO's. Given their span of control, however, we would think of them as "Captains" or even "Majors" in a modern army.
Centurions were not all equal. The top dog (Regimental Sergeant Major, perhaps, in modern parlance) was the Primus Pilus (meaning "First Spear" or "First File"). Each Cohort, except for the First Cohort, had 6 Centuries, each with its own Centurion in charge. The First Cohort had only 5 Centuries and Centurions; but also the Primus Pilus; and each Century in the First Cohort was double-strength (nominally 160 men, versus only 80 men in the other Centuries).
Centurions' specific titles also depended on which Century of which Cohort they commanded. This is complicated: if you want the full details, go to the link below, and check the charts showing individual Centurion titles for each Cohort / Century.
Below the Centurions, there were other "NCO" ranks, known as "Principales": -
"Aquilifer": The Aquilifer was the man who carried the Eagle of the Legion.
"Signifer": Each Centuria had a Signifer (so, 59 Signifers in a Legion). He was responsible for the men's pay and savings, and the standard bearer for the Centurial Signum, a spear shaft decorated with medallions and often topped with an open hand to signify the oath of loyalty taken by the soldiers. It was this banner that the men from each individual Centuria would rally around.
"Optio": Each Centurion chose an Optio from within the ranks to act as his second in command.
"Tesserarius": (Guard Commander) Again there were 59 of these, or one for each Centuria. They acted in similar roles to the Optios.
"Cornicen": (Horn blower) The Cornicen worked with the Signifer, drawing the attention of the men to the Centurial Signum and issuing the audible commands of the officers.
"Imaginifer": Carried the Standard bearing the image of the Emperor as a constant reminder of the troop's loyalty to him.
Finally ... to the Grunts: -
The basic private soldier was known as a "Milites Gregarius", or just "Milites" for short.
A new recruit was known as a "Tirones".
Various specialists, who received higher pay than the humble Milites and who were exempt from many of the drudgery duties, were called "Immunes".
And, just to complicate matters, there was a grade of wannabe apprentice-Immunes, known as "Discens". END QUOTE