Factual error: When Tom Cruise is going to defibrillate Keri Russell to disarm the bomb in her head, no defibrillator takes 30 seconds to charge to 150 joules. Most go to 360 joules and if they are not charged in 10 seconds the batteries are depleted enough that they won't charge at all.
Factual error: The walls of the Vatican are guarded by humans not cameras.
Factual error: When Ethan hangs Davian from the plane's floor hatch, Davian's head is already all the way out of the plane with the air outside blowing at him. Yet when Ethan pulls him up we find that Davian overheard Ethan's name. With the fast wind blowing at his ears he could never hear anything but a deafening roar, making Ving Rhames voice inaudible even if he were screaming.
Factual error: In the Shanghai scene, the shot immediately following Ethan's parachute getting caught on the light post shows a tall skyscraper with a pyramid peak in the background. This tower is called Tomorrow Square, located in the Jing'An District, and is nowhere near the Pudong financial district where all action is supposed to be happening. (01:29:35)
Factual error: In the scene where Davian gets hit by a truck, there is a shot of a taxi afterwards. The Chinese characters read 'bus' rather than 'taxi'.
Answer: It's never explained. This is what's known in the film industry as a McGuffin, a term popularised by Alfred Hitchcock, which is an item that's crucial to the plot, but the exact nature of which is never revealed because it actually doesn't matter in the slightest.
Tailkinker ★