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Rassilon Imprimatur is linked to (or a part of) the symbiotic nuclei which allows a Time Lord access to time travel and in particular temporal vessels such as a TARDIS.
TARDISes (or other time machines) must be primed with a Rassilon Imprimatur to enable them to travel safely in time. (DW: The Two Doctors)
The Imprimatur maps Time Lords onto the time vortex as pure mathematics. (EDA: Interference - Book One)
