The comitatenses and later the palatini were the units of the field armies of the late Roman Empire. They were the soldiers that replaced the legionaries, who had formed the backbone of the Roman military since the Marian reforms.
Units such as the Joviani and Herculiani had 5,000 soldiers and 726-800 cavalrymen. Many units' sizes would vary. There were three types of units. They were the heavy infantry, medium infantry, and light infantry. The comitatenses were the heavy infantry. The Auxiliaries, Auxilia Palatina, and the Peltasts were the medium infantry, and the psiloi were the light infantry. Comitatenses regiments consisted of 1,024 soldiers. Comitatenses legions could consist of 6,000 to 7,000 soldiers. Some of these soldiers would be lightly armed, while others would be heavily armed. During a battle the army would divide into 3-4 divisions. The army might use a double phalanx to protect its rear. Reserves would be located behind or between each division.
In the Late Roman Empire the army was divided into two units, the Limitanei and the Comitatenses. The Limitanei would defend against an invasion long enough for the Comitatenses legions to arrive. These Comitatenses legions would be grouped into field armies.This strategy has been described as "defense in depth." Once in the battle the general would do his best to avoid a pitched battle. This was done to conserve manpower. Rather than attack the enemy, the legions would form a shield wall and wait for the enemy to attack the Romans. The Romans would use their superior coordination to defeat the enemy. The Emperor would command a Comitatenses field army to put down rebellions.
Comitatenses is the Latin nominative (etc.) plural of comitatensis, originally the adjective derived from comitatus ('company, party, suite'; in this military context it came to the novel meaning of 'the field army'), itself rooted in comes ('companion', but hence specific historical meanings, military and civilian).
However, historically it became the accepted (substantiated) name for those Roman imperial troops (legions and auxiliary) which were not merely garrisoned at a limes (fortified border, on the Rhine and Danube in Europe and near Persia and the desert tribes elsewhere)--the limitanei or ripenses, i.e. "along the shores"--but more mobile line troops; furthermore there were second line troops, named pseudocomitatenses, former limitanei attached to the comitatus; palatini, elite ("palace") units typically assigned to the magister militum; and the scholae palatinae of actual palace guards, usually under the magister officiorum, a senior court official of the Late Empire.
Among the comitatenses units listed by Notitia Dignitatum there are: