Yeah, he did not specify. Barr could be taking a leave of absence for a variety of reasons: to get out of the way for Donoghue, to act as a witness in one or more matters, to spend time with his family after a hectic burst of activity...

Or to allow POTUS to appoint another AG from his available options. He probably could not get the Senate to confirm a new appointment, but as I understand it, he can re-appoint someone who has already been confirmed in the past, as he did with Bill Barr.