ENY is the value assessed each Model in the game. That eny is one of the values used in determining Perks Points awarded when one Model has destroyed another Model. The best way to explain would be to use AH's own words:
Points are awarded for both kills and damaging and destroying enemy targets. This in turn is weighted by the type of plane that you and your opposition are using. Flying the less popular planes will get you more points whether you are in air-to-air or air-to-ground mode. Likewise, the higher quality the planes of your opposition, the more points you get for them. If you shoot down lower quality planes with higher quality ones, you don't get much for doing that but you do get a lot for the exact opposite. This makes shooting down a perk plane double rewarding as you deprive somebody of a perk and get more points for doing it. Also, landing your sortie will apply a 1.25 multiplier to your perk points.
Examples:
With the values listed above, if you are flying the P-51D (ENY value of 18) and you score a kill on a Panzer (ENY value of 15), you would receive (18/15) = 1.20 fighter perk points.
Had the Panzer
scored the kill on you in the P-51D, he would receive (15/18) = 0.83 vehicle perk points.
A TBM-3 with an OBJ value of 25 that destroys a bunker with a point value of 20 would receive (25/20) = 1.25 bomber perk points.
Of course, the values used in this example may not be the current values in the game. The perk point multipliers are subject to change at any time without notice.
