The Radeon X300 Series Secondary Windows Driver typically refers to the driver that enables a secondary or additional display functionality on systems with ATI Radeon X300 series graphics cards. The "Secondary" designation suggests that this driver might be related to handling additional outputs, such as a second monitor, a secondary display adapter in a multi-GPU setup, or a secondary video channel (e.g., for dual-monitor setups, TV output, etc.).

In simpler terms, the secondary driver is required when using the Radeon X300 series graphics card for multi-display setups, video outputs, or if your system has multiple GPUs (for example, dual video cards).