The cavalry units of the Empire were used as scouts, patrols, and
couriers. Their speed and ability to cover roughly 80 km made them
an important element in the Roman frontiers. In wars, too, the
cavalry soldiers (equites)
were superior to the foot soldiers. The cavalry had a long
tradition in the Roman military and their training and supplies
were associated with high costs.
The "ala" was an independent military unit of 500 ("ala quingenaria") or 1,000 ("ala milliaria") cavalry soldiers.
The cohorts were often supplemented with 120 or 240 cavalry soldiers ("cohortes equitatae").
The legions had a relatively small contingent of about 140 cavalry soldiers.
We know about the armour not only through the finds, but also from a written source, the tract about the cavalry of Arrianus from the first third of the 2nd century..
"Reiterei", Limes-Lexikon 2009, 112f.
Bavarian State Conservation Office – Landesstelle für die nichtstaatlichen Museen & Bavarian State Archaeological Collection with the support of the Bayerische Sparkassenstiftung