Board Thread:Naruto Discussions/@comment-26479810-20171222153110/@comment-734582-20171224145456

Ninjachris wrote: How the hell could eating Kuruma FLESH give them piece of his soul that’s just plain stupid, what they got was his chakra which remained with them permanently. It was gyuki’s chakra that was contained in his tail that awakened the jubi, madara and Obito said that the “chakra” should be enough.. When Orochimaru's soul was ripped from his hands, his hands withered and died off. evidently, there's more to the soul-flesh connection than we know.

Secondly, i've already said before that there's a clear difference between a biju's chakra and a biju's chakra flesh. Kakashi isn't walking around in a Kurama avatar. The Alliance isn't. Clearly, when Kurama gave them chakra it was different than the chakra-flesh that Kinkaku and Ginkaku ate.

Thus, there's something beyond chakra to a Biju, which is what makes them Biju and not just hunks of chakra. That could be a soul for instance, but like i said, it doesn't have to be.