With a Reive Mark, you can take actions against Reive monsters and gain Reive Momentum if you possess a Reive unity. However, with the Reive Mark, you cannot improve combat or magic skills, earn experience points for individual kills, or loot fallen enemies. Additionally, you still lose experience points when KO'd.
Reive monsters do not become aggressive simply because you have a Reive Mark. You must first take one offensive action in the Reive, after which all monsters in the Reive because aggressive towards you for the duration of the battle unless all participants wipe. In other words, Reive monsters once again become nonaggressive if all hate lists are cleared (including hate lists for Lairs and Obstructions).