The Seal of Rassilon was a symbol of the Time Lords. It was a symbol of power, utilised by Rassilon during the war against the Great Vampires and against the Old Ones. (EDA: Interference: Book Two)
The symbol was also taken up by other cultures whom the Time Lords visited early in their explorations of space and time, such as the inhabitants of Voga. (DW: Revenge of the Cybermen, NA: Cat's Cradle: Time's Crucible)
The Seal was used as a symbol of identification, and something akin to deep reverence. (DW: The Five Doctors, Doctor Who: The TV Movie)
When he sent the Master into the Death Zone to rescue the Doctor, the President of Gallifrey gave the Master a copy of the seal, hoping it would convince the Doctor of the Master's good intentions. (DW: The Five Doctors)
At some point, the Time Lords also used a different seal, bearing Gallifreyan markings. (DW: The Sound of Drums)
