I dunno about all the other games, but, I think it means that you can make a Guilty Gear using Guilty Gear characters and their signature stuff instead of having to make a spinoff (blazblue) with characters that look lie the characters you wish were your characters.
Yes.
Confusing.
Simply: I am pretty sure they made BB because the GG characters were not owned at the time and were warped up in red tap. Now that they do own them, they might let us play with them. Just my speculation though.