Many frames designed for extreme riding are built with an 83mm bottom bracket shell. It's a result of the wider axles used to optimize wheel stiffness to improve durability. The extra width in the back necessitates extra width in the cranks to get a proper chainline. If you have an 83mm shell and plan to run a SRAM/Truvativ GXP crankset, this is the bearing set you'll need.
SRAM's GXP bottom brackets are brand specific -- meaning, they can only be installed with SRAM type cranks. The key element that makes them different from a Shimano or FSA bottom bracket is that the left side bearing has a smaller diameter to mate with the stepped spindle of the SRAM crank. This design allows the inner race of the left side bearing to be captured between the crankarm and the spindle, effectively locating the crank. The right side bearing "floats" on the spindle handling only radial loads as you pedal. The SRAM design optimizes bearing load, minimizes drag, and gives you durability.
The GXP 83 Bottom Bracket has alloy cups and sealed cartridge bearings. It's compatible with Truvativ GXP mountain cranksets designed for an 83mm bottom bracket shell.