From Wiki:
JPEG XL supports lossy compression and lossless compression of ultra-high-resolution images (up to 1 terapixel), up to 32 bits per component, up to 4099 components (including alpha transparency), animated images, and embedded previews.
Why 4099 components? Why so many? And why 4099 in particular? 4096+3 with 3 being RGB?
On a side note, 1 Terapixel is just crazy. A square with 1 million pixels has this number of pixels. So, about 1000 of 1080p will fit into this square vertically and about 500 horizontally. How has such eyes to see this all pixel perfectly?