The Radeon X600 series secondary driver refers to a specific driver component that is often required for the proper functioning of dual-display setups or systems with multiple graphics outputs (i.e., using multiple monitors or secondary graphics cards) with an ATI Radeon X600 series graphics card. The "secondary" part usually refers to additional or secondary video outputs, such as a second monitor, TV-out, or even a secondary graphics card used alongside the primary GPU.