ENY means when we eez reeely beezy in zee ahreena, you dozent gets eny pleenz.
Or in readable English, when too many folks get on in one country, to keep things from getting lopsided in favor of that country with all those people in it, they start making certain planes unavailable. As you get more people, the lower ENY value planes are taken off the list. You try to take a SpitV for example, and it will tell you the current ENY value is greater than that of the plane you want. Most perk planes are a 5 ENY, which means they are among the first to become unavailable.
Its designed to balance things out so that no one can get an unfair advantage.