ENY was originally (and still is) a system of calculating perk points whereby, if you shot down a low ENY plane with a high ENY plane, you score more perk points than doing the opposite. In effect, it was (is) a rudimentary plane ranking, with lower ENY planes being "easier" to fly and fight in than higher ENY planes.
At some later point in time, it was deemed that, with lopsided country balance in the arenas, the country(s) with the lowest numbers needed help and ENY was used as the means to attempt to achive balance. Now, when the arenas become significantly imbalanced, an ENY limiter kicks in for the country(s) with the highest numbers, limiting the players to a minimum ENY planeset (the lower ENY value planes, those deemed "best" are eliminated from play for that (those) country(s)).