I don't think it means a weapon that can only be wielded by one class just a weapon that is significantly more powerful to a certain class, e.g. The hunter could have a sniper which could cause precision hits (even if the enemy wasn't dead) to explode and over penetrate through 5 guardians but if a titan used it it wouldn't cause an explosion just over penetrate but have it penetrate through say 3 people (I know it's a bad example but it is just so you get the idea, the weapon is less effective in the hands of another class)