User blog comment:B214/Volume 23 discussion blog/@comment-77.188.220.109-20170502124957/@comment-30924344-20170502134020

Well he won't be able to use his ExE form then. AxA will be Great Red's power whereas ExE is using both Great Red's and Ophis' power. Issei needs to master DxD and AxA to use ExE.

I think the meaning of AxA will be this: Ambitious Apparition. Ambitious refers to all the things Issei wants to achieve for the people around him. Apparition refers to Issei's remarkable/unexpected appearance as a huge dragon.