Ultimate Void

According to one account, most direct battles in the Last Great Time War were fought not in physical space, but in the Time Vortex and "even beyond", in the Ultimate Void. For this reason, most of the lesser species knew nothing of the War, (PROSE: Meet the Doctor) except in those occasions when a corner of the War would briefly bleed out into conventional space, wreaking untold destruction within instants. (PROSE: Revenge of the Nestene) Other accounts of the conflict did show that the war could be fought on terrain (COMIC: The Clockwise War) or in normal space. (PROSE: Engines of War)

Behind the scenes
Much as Russell T Davies' Meet the Doctor features a mention of long before it was first uttered on-screen in Davies' televised series, and in a form not especially consistent with the way the concept was presented upon finally making it to the screen, it seems that the Ultimate Void is an earlier "draft" of the Void, which would play a major part in the finale of Davies' Series 2. However, the capitalised "Ultimate" was never appended to the Void in this later usage, nor was any link drawn between it and the Great Time War's battlefields; indeed, the Cult of Skaro escaped the Time War by hiding out in the Void.

As such, even though the behind-the-scenes echo is obvious between the two Voids written into Doctor Who's cosmology by Davies, there is no in-universe evidence that they are one and the same, and, indeed, much to call such a notion into doubt. The nature of the Ultimate Void from 2005 thus remains in doubt.