The Rassilon Imprimatur was a code engineered by Rassilon into Time Lord biodata. It was responsible for mapping Time Lords onto the vortex as pure mathematics. (PROSE: Interference) It was impossible to travel in a TARDIS without the Imprimatur. (PROSE: Divided Loyalties)
With the Imprimatur, Time Lords could put themselves into different time states just by thinking about the right equations; at the Academy, trainee Time Lords used it to transmigrate objects, though the Cardinals said that abusing the Imprimatur in that way "played merry hell" with one's biodata. The Eighth Doctor used it to escape from prison. (PROSE: Interference)
The Sixth Doctor claimed that the Rassilon Imprimatur was a symbiotic print in Time Lord physiology. Timeships like TARDISes and the Kartz-Reimer module needed to absorb the Rassilon Imprimatur through their briode nebuliser in order to prime them to allow anyone to use it. However, the Doctor lied about at least part of this description to trick the Sontaran Stike, who was listening nearby. (TV: The Two Doctors)