ILFORD XP2 Super is a chromogenic (colour-forming) film - one on which the final image is made of coloured dye, not metallic silver.
During processing, the developer reduces exposed silver halide grains to metallic silver, and the oxidised developer reacts with chemicals known as colour couplers to form dyes. The developed silver is subsequently removed by bleaching.
The chromogenic process was originally designed for colour photography. The first commercially-available black & white film to use it was ILFORD XP1, whose current successor is ILFORD XP2 SUPER.
Chromogenic films must always be subjected to the appropriate colour process - in the case of XP2 SUPER this is C41, which widely used for commercial processing of colour negative films. Conventional black & white processing of XP2 SUPER will produce only a silver image, which will be inferior to the dye image given by C41.
When the colour couplers used in XP2 SUPER react with the oxidised developer, they also release development inhibitors. These hold back the development, especially in strongly exposed areas. The result is particularly high sharpness, and much lower graininess, compared with conventional films of the same speed. In addition, graininess is reduced by overexposure - the opposite of what is found with conventional films.
