With the launch of Stormblood

[Lyth]

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Not really. MT and OT are designations which refer to which tank is actively tanking at a given point in time. There are plenty of situations in which the group benefits from having the WAR actively tank, depending on other factors such as unchained on pull, cooldown timings, any active debuffs, and so on. There is no clear line. You have to look at it on a case-by-case basis and play to each of your strengths. The devs give you the tools, but it's up to the player to make the maximum use out of them.

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The game does divide different jobs into different categories. However, whether the devs intend to create a subdivision within the tanking role remains to be seen. MT/OT designations are extremely fluid, and players are adhering less and less strictly to them. For that matter, if you're actively raiding, then you'll also know that WARs generally tend to be the designated "MT" at the onset of fights because of unchained. MT/OT designations were player-enforced from the outset. The bit about duplicate jobs in RF is irrelevant to this discussion.