Used to get into the New Vegas Strip.
This can be obtained by completing the G.I. Blues for the Kings, located in Freeside. On completion of the quest, The King gives the player a couple options for a reward, one of them is to get a free passport at Mick & Ralph's store, otherwise the passport is 500 caps (this option to buy the passport without using the King's favor is only possible if a Speech check of 50 is passed). With high enough Barter skill, there is an option when talking to Ralph to lower the price to 375 caps.
However, the player can still get into the Strip for the first time as long as they can pass the credit check of 2000 caps. Note that the player does not lose the caps, they just need to be shown.
A passport is not required if the Strip is entered via the monorail at Camp McCarran or with Science above 80; in this case the Securitron can be convinced to allow entrance.