This typically signifies content or a specific category of run that is limited to a certain platform (like a PC-only glitch) or a specific community event (like a "Speedruns Live" exclusive race). Essay: The Convergence of Precision and Velocity
Rumors from the official roadmap (leaked via a Twitter DM screenshot) suggest that version 2.0 will introduce a mode. Details are scarce, but dataminers found references to: pixel speedrun 6x exclusive