Biodata module

A biodata module (COMIC: The Promise) was Time Lord technology which, used in conjunction with a Chameleon Arch, could modify the biology of an individual, essentially changing that individual's species while a TARDIS could program the individual's memories to allow them to fit in better.

Gallifreyan symbols were commonly inscribed on a module's external surface, which in rare cases caused it to be identified. (TV: Utopia)

Functionality
The module, often shaped like a fob watch or a similar object, was placed on the front on the Time Lord's forehead and stored the Time Lord's essence. When opened, it would return it to the Time Lord. (TV: Human Nature / The Family of Blood) The Eighth Doctor claimed that destroying a Chameleon Arch while the user was not a Time Lord would render their Time Lord personality and biology irretrievable. (AUDIO: One Life) The biodata module was incorporated with a perception filter to ensure that it would not be noticed. (TV: Human Nature / The Family of Blood, Utopia)

Modules resembling fob watches were activated by opening the lid. (TV: Human Nature / The Family of Blood, Utopia) The module containing Ruth Clayton's true identity, appearing to be a fire alarm, was activated by breaking its glass. (TV: Fugitive of the Judoon)

History
Quarren Maguire used a Chameleon Arch to escape the Last Great Time War, storing his essence in a locket-shaped biodata module. (AUDIO: One Life)

also used the Arch to turn into a human to escape the Last Great Time War. His module was shaped like a fob watch. (AUDIO: The Heavenly Paradigm, TV: Utopia)

The Ninth Doctor gave a biodata module to Plex, modified to only copy the user's mind, without giving a new one. It was later retrieved by the Twelfth Doctor, destroyed and without any remaining biodata. (COMIC: The Promise)

The Tenth Doctor used another biodata module to change himself into John Smith to escape from the Family of Blood. He later gave it to Timothy Latimer. (TV: Human Nature)

The Tenth Doctor used a representation of his Chameleon Arch to recover his memories, after his mind was taken over by Es'Cartrss of the Tactire. (COMIC: The Forgotten)

During his eleventh incarnation, the Doctor kept a biodata module in his store room aboard the TARDIS. (GAME: TARDIS)

Ruth Clayton used a biodata module which resembled a fire alarm affixed to the wall of a lighthouse, restoring her identity as an unknown incarnation of the Doctor. (TV: Fugitive of the Judoon)