This article on the official site does absolutely nothing to clarify the situation.

Speaking of her return, Billie said: “It’s no secret how much I love this show, and I have always said I would love to return to the Whoniverse as I have some of my best memories there, so to be given the opportunity to step back on that TARDIS one more time was just something I couldn’t refuse, but who, how, why and when, you’ll just have to wait and see.”