The seal of the High Console of Gallifrey.
I Nicked it off the Master in the Death Zone.
There is an algorithm imprinted in the atomic structure, use it to decode the message.
It is a reference to "The 5 Doctors" episode. They TimeLords traded the Master a complete new regeneration cycle to rescue the Doctor from the Death Zone on Gallifrey. They gave the seal of the Timelords to the master to prove to the Doctor that he was working with them. Jon Pertwees Doctor took it from the Master and said he was going to return it to the Timelords which apparently did not happen.
I am guessing they tied that in because they had already set the rules back then for the possibility of a new set of regenerations so they did not really need to explain anything else. At some point they can just say that the new life cycle was somehow stored in the seal the whole time and was just never activated (or something like that

lol). IT is an easy way to explain the regen's without having to alter whatever cannon is already there. Problem Solved, blame John Nathan-Turner

lol

lol.