A Diplomatic TARDIS was a model of TT capsule that Susan Campbell noted were nearing obsolescence when she lived on Gallifrey. During the Last Great Time War, all TARDISes were called out from storage.
Susan, Ian Chesterton and Veklin used a Diplomatic TARDIS to travel to the Sense Sphere. Ian described the console room as being like a cathedral compared to the First Doctor's TARDIS. (AUDIO: Sphere of Influence)