Rassilon Imprimatur

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.

TARDISes (or other time machines) had to be primed with a Rassilon Imprimatur to enabled them to travel safely in time. (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)