Rassilon Imprimatur

The Rassilon Imprimatur was linked to (or a part of) the symbiotic nuclei which allowed a Time Lord access to time travel and in particular temporal vessels such as TARDISes.

The Sixth Doctor once claimed that TARDISes (or other time machines) had to be primed with a Rassilon Imprimatur to enabled them to travel safely in time. He said this knowing Stike was nearby, to fool him into letting the Doctor go for a trip in the Sontarans' Kartz-Reimer module. (DW: The Two Doctors)

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

When a Gallifreyan was granted the rank of Junior Time Lord in the Time Lord Academy, they received their symbiotic nuclei. (EDA: Interference - Book One)