From TF2 Wiki
The Intelligence, also known as the Intel or Briefcase, is the eponymous flag and objective of the Capture the Flag game mode. During a Capture the Flag game, it must be retrieved from its designated location and returned to a defined point. It appears as a spinning briefcase in the color of the team to which it belongs.
A player may pick up the intelligence by simply walking into it. While carrying the intelligence, a player cannot use teleporters or become ÜberCharged. Scouts are also unable to drink Bonk! while holding the Intelligence, and may not take it while under its effects. Spies are unable to pick up the Intelligence while cloaked, but are able to pick it up while in disguise (which will automatically remove it). However, they cannot disguise or cloak while they are carrying the intelligence, and will automatically drop it if they do.
The Intelligence is dropped automatically if the carrier is killed or enters a resupply area. It can also be dropped manually if the player carrying it presses the drop-item button (Default key: "L"). Once dropped, a clock-like timer ring will appear over the Intelligence. When 60 seconds (default) have passed without an enemy picking it up again, it is automatically returned to its original position. As there is no way to forcibly return it to base, it is crucial that a team defend their dropped Intelligence during this period. If sudden death occurs, the Intelligence is automatically restored to its original position.
- As placeholders during early stages, Valve used modified versions of Team Fortress Classic's flags, which can be still be seen in the game files.
- When carried on a player's back, the Intelligence will leave a literal 'paper trail'.
- With the Classless Update, intelligence visuals when being carried by a player were improved to make detecting the carrier easier.