The E is for Extreme. The EXPS3 offers true 2-eyes-open shooting and a 7 mm raised base that still allows for iron sight access. It features easy-to-adjust side buttons and an adjustable" locking" quick-detach lever for easy attachment and removal. It is also the optic of choice for the technologically advanced modern rifle hunter. Its night vision-compatibility makes it a versatile choice for day or night hunting.