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' experimental time capsule. (DW: The Two Doctors)

The Imprimatur mapped Time Lords onto the time vortex as pure mathematics.

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)

Behind the scenes
The only mention of the Rassilon Imprimatur on-screen was in The Two Doctors. Due to the nature of the scene it is unclear whether the Doctor is honestly describing an actual technical requirement of the Kartz-Reimer module, or lying to ensure Stike forcing him to "prime" the module (and granting him the opportunity to covertly sabotage it.)

Spinoff mentions of the Rassilon Imprimatur have generally assumed the Doctor's statement to have been entirely true, and treated the concept thusly.