You are partially correct and incorrect. You unlock weapon skins, armor skins, and certain armor as you level through the first 1-50 levels. You than unlock Specialization specific weapon skins, armor, and armor skins as you go through the 1-10 levels for each Specialization. At the end of each Specialization you also unlock the MOD specific to the Specialization. The skins, armor, bonuses you get for pre-ordering are available when you input the codes.