Stages were the humanoid species' way of mastering control over the large amount of power that came with the progression of cores.
With each advancement from one Stage to the next, significant boosts would be attained, making humans and others creatures of similar nature, stronger than beasts.
The first Stage was the Foundation, where one laid down a solid blueprint for their progression. Next was the Advancement, where some were likely to develop Advanced classes but the probability was relatively small.
At this Stage, individuals could awaken a Full Body Aura. It was a concentration of their mana which gave boosts to their already existing attributes and skills.
Benzard, Denille, Irlen and Reon were at this stage. All four had awakened their Full Body Auras, but as Benzard had said, they had barely grown out of their Foundations, unlike Eobald who had been halfway through the Advancement Stage.