Wikipedia
The limitanei or ripenses, meaning respectively "the soldiers in frontier districts" (from the Latin phrase limes, meaning a military district of a frontier province) or "the soldiers on the riverbank" (from the Rhine and Danube), were an important part of the late Roman and early Byzantine army after the reorganizations of the late 3rd and early 4th centuries. The limitanei, unlike the comitatenses, palatini, and scolae, garrisoned fortifications along the borders of the Roman Empire and were not normally expected to fight far from their fortifications.
The limitanei were lower-status and lower-paid than the comitatenses and palatini, and the status distinction between scolae, palatini, comitatenses, and limitanei had largely replaced the older distinction between praetorians, legionaries, and auxiliaries. The limitanei and palatini both included legionary units alongside auxiliary units.
The nature of the limitanei changed considerably between their introduction in the 3rd or 4th century and their disappearance in the 6th or 7th century. In the 4th century, the limitanei were professional soldiers, and included both infantry and cavalry as well as river flotillas, but after the 5th century they were part-time soldiers, and after the 6th century they were unpaid militia.
The role of the limitanei remains somewhat uncertain. Hugh Elton and Warren Treadgold suggest that, besides garrisoning fortifications along the frontier, they operated as border guards and customs police and to prevent small-scale raids. They may have driven off medium-scale attacks without the support of the field armies. Edward Luttwak saw their role as a key part in a strategy of defence-in-depth in combination with the provincial field armies