definition
Eye relief refers to the distance of ones eyball to the face of the rear lens, in which a full sight picture can be maintained while looking through the scope. By sight picture I mean full use of the field of view, with no blacked out edges or diminishing of the "window". As a rule a lot of older scopes were a bit less forgiving, newer scopes better. I have a K12 that has a very limited eye relief range. I can tell a difference between my Burris Fullfields (USA) and most of my Leupold of equal value, the Leuy's being more forgiving. I've only been smacked by a scope once, a full length 4x Tasco on a 3"-12 ga mag, for a turkey gun experiment. That convinced me that combo was a no-no, but a Leupold 2.5x compact worked well on my gun and a pals. Those little Leupolds are advertised as generous, and they are indeed.
BTW, scout scopes are typically "intermediate" eye relief. Pistol scopes are "long" . Some pistol scopes will work when mounted in the intermediate "scout" position forward of the action. But a IER scope will usually NOT work on a handgun.