Ok getting this question all the time so
As you all should know by now buffs are 6 diff types (series).
If you are not class rank 15 or higher u can have only 1 buff of the same series.
What this means is that for example archers cant use their regen and hp+ buffs at the same time anymore. But warriors and bladers can use Iron skin and Aura barrier together as soon as they get both.