Quick answer
Conventional XLPE is rated for a maximum continuous conductor temperature of 90°C. High-ampacity XLPE uses an insulation suitable for 105°C. The 15°C difference matters because a cable's current rating is set by the gap between ambient temperature and the conductor limit — widening that gap lets the same cross-section carry up to 15% more current. The benefit is largest exactly where ambient is hottest: at a 50°C ambient a 90°C cable is derated to about 89% of its catalogue rating, while a 105°C cable keeps its margin. See high ampacity XLPE cables.