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 late republic.
The limitanei would deal with smaller raids, or, in the case of larger invasions, try to defend or stall long enough for the comitatenses legions to arrive.
[3][4][5] Comitatenses is the Latin nominative plural of comitatensis, an adjective derived from comitatus ('company, party, suite'; in this military context it came to the novel meaning of 'the field army'), itself derived from 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.