Biodata module

A biodata module (COMIC: The Promise) was a Gallifreyan technology which, when 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 the Fugitive Doctor. (TV: Fugitive of the Judoon)

The Division kept a biodata module containing the Doctor's memories from their time as a Division operative aboard their space station in the Void. (TV: Survivors of the Flux) The Thirteenth Doctor managed to claim the module but decided against opening it, trusting the TARDIS to hide it somewhere the Doctor would never find, unless she "really ask[ed] for it". (TV: The Vanquishers)

Behind the scenes
In The Forgotten, the Eighth Doctor claims that he was only half-human during the events of the TV Movie due to having used a "half-broken Chameleon Arch" on himself to fool. Given the prominence of shots of the Doctor's fob watch throughout the TV movie, which were not intended to serve a specific plot purpose other than emphasise the Doctor's association with time, the implication seems to be that this watch was actually a biodata module. Other fob watches, including one even more closely resembling the design used in Series 3, can also be seen inside The Doctor's Gladstone bag. However, the connection is not made explicit. Additionally, many other accounts present different interpretations of the Doctor's infamous "half-human" comment, with the Chameleon Arch theory only being one possibility.