From Halopedia, the Halo wiki
A flex card is an archaic form of human currency that remains in circulation on Venezia. Small in size, a flex card is a slip of bright orange plastic. A piece of a flex card can be broken off along a stress line and used for spending. Flex cards remain in circulation on Venezia as physical currency is desirable over an electric banking system which would be subject to power outages due to the colony's outdated technology. On Venezia, a flex card is exchangeable for several alcoholic beverages from any bar in New Tyne.
In 2553, Venezian Militia member Nairn gave a flex card to Vasily Beloi and Naomi-010, who were undercover acting as UNSC defectors. The two later spent the card at a bar to buy beverages and appetizers at Stavros' Middle East Asian Cuisine and Bar.
List of appearances