It is a rather silly term for us, isn't it? Then again, he could have just as well used "personality" or "persona", but I suppose what he's getting at are each separate will is a person, not just a personality or a persona, but he needed a simple term to designate it as different from the sum of all the people in an entire body. Perhaps instead of person and subperson, it could have been system and person, but considering the entire book has this one paragraph expounding on Hofstadter's theory, and the rest is on general about the various theories concerning the mind, I doubt he couldn't really go into it.