This term is actually a combination of how far from the rear of the scope, and how wide the size of that sweet spot is. As you increase magnification, the eye relief can shrink. This wasn’t so much of an issue when the high zoom scopes generally only lived on the small-bore and varmint rifles. But this has changed – now, magnums and more are wearing high zoom scopes to enable them to reach out even further.
So – as scopes have become more and more powerful, more and more eye relief (at the top end) has become a requirement.
Short eye relief, lots of recoil – a perfect recipe for some scope bite.
However, while this is an import factor, it works in with another, sometimes misunderstood term – field of view.