Whereas it may be used in outdoors-like natural lighting, it is not (always) true that it is "usable in the sun". On one such product¹ there's this "Important Notice" advising users to "avoid exposing the E-ink screen to direct sunlight or intense ultraviolet rays, as this may cause irreversible damage to the screen."

¹ https://shop.dasung.com/products/dasung-25-3-e-ink-monitor-p...