Revan was a Sith Lord and the namesake of the Sith Eternal army’s 3rd Legion. Since the history of the Sith was kept hidden from the galaxy, only Sith cultists knew the significance of Revan’s name.
Sith trooper legions were numerically identified, yet also named after an ancient Sith Lord. The Revan Legion was activated in 35 ABY along with the rest of the Sith Eternal’s forces during the war between the First Order and the Resistance.