From Traderpedia

Definition: Trading pit abbreviation meaning "Contract"

Carried from long ago, from grains at the CBOT, in which a "Car" was a contract that represented one railroad cargo car -- and still does. Traders say it instead of a "Contract", as slang, since its one syllable, and as the professionals know, is the preferred terminology on the trading floor. To recap: "XYC Co sells 100 Cars not held" means they just sold 100 Contracts with broker discretion to execute the order at the market.