Type 40

The  — or, formally, the Type 40 time travel capsule — was the class of TARDIS to which the Doctor's TARDIS belonged. (TV: The Deadly Assassin, et al.) In comparison to the models used by some other renegades and other Time Lords who went beyond Gallifrey on official missions, it was old and outdated, and was considered to be a museum piece or scrap.

Information
Little was actually known about the type, aside from the Mark 3 example provided by the Doctor's TARDIS herself. (TV: The Time Meddler) The later Mark IV model had several features that resulted in smoother operation than the Mark 3 but it had a higher energy consumption rate than its predecessor, along with some of its hardware being incompatible with the earlier model. (AUDIO: The Resurrection of Mars, The Book of Kells, TV: The Daleks' Master Plan)

The Type 40 was intended to have six pilots to operate the console smoothly. (TV: Journey's End)

Obsolete
The one quality on which most of the Doctor's Time Lord contemporaries agreed was that Type 40s were antiques. At one time or another, (TV: The Claws of Axos) and  (TV: Time and the Rani) all described the Doctor's TARDIS as undesirably old. Romana I indicated that the type was no longer on the syllabus at the Time Lord Academy. (TV: The Pirate Planet) Indeed, once when the Fourth Doctor returned to Gallifrey, Commander Hilred called the type "obsolete" and noted that all had been pulled from service centuries earlier as more advanced models were introduced.

Chancellor Goth was amazed a Type 40 was still operational, though when he inspected the Doctor's TARDIS he admitted it was in good condition. (TV: The Deadly Assassin, The Invasion of Time) Professor Chronotis, who was clearly of an older generation than the Doctor, claimed that Type 40s first came out when he was a boy. (TV: Shada, WC: Shada) When the TARDIS inhabited Idris, it mentioned to the Doctor that it was already a museum piece when the Doctor was young. (TV: The Doctor's Wife) It seems, however, that while they were decommissioned, they were not all destroyed, as another was activated by Straxus to stand in for the Doctor's. (AUDIO: X and the Daleks)

Coordinator Narvin considered the Type 40 to be scrap compared to the far superior Type 400 model. (AUDIO: The Quantum Possibility Engine)

The Eighth Doctor loved his TARDIS, in part, for coming from a "classic line". (AUDIO: The Scapegoat) Indeed, River Song defined the Type 40 as a "vintage model". (COMIC: Pond Life)

Other distinctive qualities
Exactly 305 were said to have been grown and officially registered with the Gallifreyan authorities. (TV: The Deadly Assassin) All were installed with the Record of Rassilon. According to Romana II, only the Type 40s were so equipped. (TV: State of Decay)

Behind the scenes

 * The Doctor's TARDIS was not always referred to as a "Type 40". Initially, there was no suggestion that the TARDIS was anything other than a unique craft. However, with the broadcast of The Time Meddler, the notion was seeded that, like cars and other devices, TARDISes were produced with mark numbers, as the Monk's TARDIS was a Mark IV, with the Doctor's not named in the serial. The actual type numbers of other Time Lords' TARDISes have never been given in televised stories. However, other type numbers have been mentioned in other media. More recently, the Doctor's TARDIS has been given the designation TT Type 40, Mark 3 by the onboard computer on the Teselecta in the episode Let's Kill Hitler.
 * In the Apple Arcade game Hidden Mysteries, the Doctor's TARDIS was referred to as a Type 44.