It was essentially a wafer of molecular circuitry, embedded inside an indestructible plastic mantle. It contained a bank validation code, a numerical value, and a high-end security key; any attempt to change the programmed value would trigger the security code, causing the invalidation of the chip.
Credit chips were issued by interstellar monetary institutes such at the Banco de Madrid, and were recognized as legal tender anywhere in human-inhabited space. Unlike most other forms of modern money-handling, the chips were all but untracable, making them a favorite of criminals such as slave traders. (CS3)