In an armed force, a standard is a banner, ensign, or figure, carried along on a pole by a bearer as a symbol that identifies or distinguishes a military unit, conveying values, such as honor and courage, and whose members must follow or rally to. In the Roman Army, there were basically six standards; the Aquila (Eagle), the Vexillum, the Signum, the Manus, the Imago, and Draco.
The Aquila was the symbolic standard of the Legion and was carried when the Legion marched on a military campaign. Its bearer was called aquilifer, who wore a lion skin over his head.
The Vexillum was a square piece of red cloth, with golden fringes, with a figure of a wild animal in the center and the Legion number above. Fixed on a crossbar on a long pole, it represented what legion a Roman soldier belonged to and which he had to follow. It was carried by the vexillarius, who wore a wolf skin on his helmet.
The Signum was the emblem of the cohort and consisted of a series of six metal disks on a pole, topped off by a hand. Each of the disk represented a century. It was born by the signifer, who also had a wolf head skin over his helmet.
The Manus represented a maniple unit and was composed of two disks on a long pole, also topped by a hand. Two disks, because the maniple was made up of two centuries.
The Imago was the symbol of the empire and consisted of the emperor head on relief on a piece of metal on a pole. It was carried by the imaginifer, who wore a bear head skin over his helmet.
Draco was a dragon head on a pole and was the symbol of a cavalry unit.
Down below the golden Aquila in marble relief from ancient times
Below, two Signums, one Vexillum, and the Eagle can be seen in a 1st century AD relief