When you check in HANGAR, each plane has a ENY value. The lower the ENY, the "higher performance" the plane is.
The amount of perks you get awarded for a kill also depends on the ENY value of both planes. The higher your own planes ENY and the lower the opponents, the more perk points you get. Also into play comes the perk multiplier which gets calculated from how many people on your country are in flight as compared to the other countries.
ENY *limit* was installed for side balancing reasons. Above a certain total number of people online, the limit takes effect if one country highly outnumbers the other two. At one point, HT posted the formule used to calculate the limit. The stronger the imbalance, the higher the "ENY limit" ie you cant up planes below a certain ENY value.
This means the country that has the numerical advantage gets a "handicap" in plane choice. Also, not being able to fly the plane of your choice makes people either change country or log alltogether, which in turn helps evening the numbers.
As you can imagine, the higher the total number of players, the better the system works (for mathematical reasons... one single player doesnt make that much of a difference). With the new arena setup, the number of players per arena is lower, closer on the edge of minimum numbers needed to make the system (as it is now) work. This might lead to extreme turnouts of eny limit, which is cause of lot of gripes - of course. HiTech repeatedly has stated that hes working on tweaks to make the system more effective and smooth again.
That some sort of side balancing tool is *needed* was shown in the EWA, where the eny limiter was turned off for a while, resulting in 40 players on one country beating on 12 players (on *both* other countries!) with perked rides.
