Rassilon Imprimatur

The Rassilon Imprimatur was ostensibly a part of a Time Lord's biology. Much of the purpose of the Rassilon Imprimatur was stated by the Sixth Doctor, who admitted to Jamie McCrimmon that none of the description the Doctor gave to him was strictly true; the Doctor said what he did because he knew the Sontaran Stike was in the vicinity. (TV: The Two Doctors)

The Doctor claimed within earshot of Stike that the Rassilon Imprimatur was a "second trick up [the] sleeve" of Rassilon. He said that a time machine such as a TARDIS or the Kartz-Reimer module (which copied the technology of a TARDIS) had to be primed by the Rassilon Imprimatur, which the Doctor referred to as a symbiotic print in the physiology of a Time Lord, once the briode nebuliser absorbed it. The Doctor claimed that this made it into "a time machine that anyone can use". (TV: The Two Doctors)

The Imprimatur mapped Time Lords onto the time vortex as pure mathematics. (PROSE: Interference - Book One)