Stupa is a symbolic representation for the level of awareness of humanity. It is in just the same way as the mountain of awareness presentation. As a member of humanity become more spiritually evolved. He will become less dense with a finner vibration. Accordingly then his level of awareness or consciousness will rise. Lower humanity is at the bottom. Middle section is for average humanity. Top section is for higher man; who eventually become lost to the material world as a fully dissolved being.