Interstitial time was an envelope of non-space, non-time, underlying the "real" universe and surrounding and separating every space-time event. The inside of the Doctor's TARDIS was considered outside of the real universe, (PROSE: Falls the Shadow) described as not being in local spacetime. Vilal vortex ships existed in the time vortex, on the "edge" of spacetime. (AUDIO: Quicksilver) The granularity of space-time allowed the Master to utilise interstiality and pull objects through time. (TV: The Time Monster, PROSE: The Quantum Archangel)
Behind the scenes
In Iris Wildthyme and the Unholy Ghost, the term "tempero-spatial" was used, which in the real world is a genuine term to describe spacetime, but this definition was never given within the story.