a long long time ago it was possible to spawn items which significantly higher rolls. However these no longer drop and therefore are called "unspawns".
Overspawns can only be found with magic luck, other than an interesting case where there's a 16 EES item running around:
https://forums.d2jsp.org/topic.php?t=84528302&f=273 (max including the expected "+1" overspawn would be 15)
So just 1 tier higher, so tier 3's can get overspawn with tier 4 stats. Any item which is higher than this is an unspawn.