A numerical Type designates each model of TARDIS. Every TARDIS that is constructed follows the specifications of its specific "Type." For example the Doctor usually operates a Type 40 TARDIS. Higher Type numbers indicated later model TARDISes. The Type number is fixed. For example the Doctor's TARDIS is still a Type 40 TARDIS despite the numerous upgrades and modifications it has received over the years. If a TARDIS Type is particularly successful, then over 300 of that Type might be created. The Type 89 TARDIS is the most advanced non-military TARDIS yet created. Military TARDISes are known to go up to Type 94.

The fully sentient and interactive TARDISes created during the Time War are some times called Type 100s (Type 101, 105, etc...) but are more properly referred to a Form 101 or Form 103 etc... The Form 105 is the most advanced TARDIS known to exist.

The Mark number is much more variable. It denotes the current sophistication of the Master Control Console (and, by extension, many of the other peripheral systems). Thus a TARDIS could be a Type 35 Mark I or a Type 35 Mark III. Both TARDISes were originally Type 35s but a majority of their systems have been upgraded to the level of later Types. A Time Lord would find that all Mark III TARDISes (regardless of their Type) have most of the same systems and extras.

So Type designates the original model as produced by the Neural Construction Docks. Mark designates the current capabilities and level of sophistication of the TARDIS in question.