User blog comment:B214/Question and Answers 6/@comment-2607:FB90:603D:5340:4082:7A47:2AEA:26C5-20190112030547/@comment-10330822-20190130030315

Touki can be used by any being that trains to master their life force that way. Senjutsu/Yojutsu users also seem to have an edge in using it, because they manipulate base life in general.

Dragon Aura, is a characteristic specifically of dragons and their offspring. Issei possesses a Dragon God Aura, but his children seem to have inherited a dragon aura of their own, which makes them much stronger. How exactly it functions in never specified, but when Issei uses his Dragon God Aura, its almost as if he is channeling the power of his DxD form into his punches and other attacks.