Emergency Program One

Emergency Programme One was a feature of the Doctor's TARDIS designed to return a companion to a designated place in case of extreme emergency (DW: The Parting of the Ways)

The Tenth Doctor mentioned that Emergency Programme One was automatically activated after five hours. (DW: Silence in the Library) When activated, it projected a hologram recording which explained the programme. (DW: The Parting of the Ways)

Uses

 * Believing that he would die on the Game Station, the Ninth Doctor used it to play a message for the benefit of Rose Tyler before the TARDIS returned  Rose to her home of Powell Estate in 2006. (DW: The Parting of the Ways). The message mentioned Rose by name, and The Doctor's hologram (at the time of recording) didn't know what specific emergency would cause him to activate it but described it as "facing an enemy who should never get their hands on this machine" (referring to the TARDIS itself).
 * Whether The Doctor recorded another similar message after his regeneration or for any of his other companions (such as Martha or Donna) is never revealed, but the Tenth Doctor's reference to the program means he probably had (or else Donna might have been faced with a man she didn't recognise talking to someone else). A hologram of the Tenth Doctor similar to Emergency Program One's Ninth Doctor hologram appeared in DW: Blink, but this was called Security Protocol 712 by the hologram itself.
 * Donna Noble recorded a message on it as a way to say good-bye to the Tenth Doctor, in the event that she might not have a chance to do so herself. (DWM: The Time of My Life)