Currently, the only available information about the pylon's UIposition config entry is "UIposition[] = { 0.1, 0.25 }; // x,y coordinates in 3DEN UI". (found here)

From that line, it would be expected that UIposition is an offset (from the top left) based on the percentage of the aircraft picture's width/height. However, this is not the case. For example, for the AH-9 Pawnee B_Heli_Light_01_dynamicLoadout_F, the Y coordinate for the UIposition of both of its pylons is 0.4. But from visual inspection, its easy to see that the combo boxes for the pylons definitely appear on the lower half of the image (more than 40% of the way down).

To further demonstrate this, I changed one of the pylon's UIposition to {1, 1}. The ctrlPosition of the aircraft picture was [0,0,0.739286,0.554762] and the combo box's was [0.924107,0.924603,0.162698,0.0396825]. If this worked as expected, the combo box's X and Y positions would be expected to be exactly the picture's width and height, respectively.

Clearly, there is something else going on in the engine's handling of the 3DEN pylons attribute in terms using the UIposition config entry. This has lead to a frustrating experience when trying to implement a pylons configuration UI similar to the one available in 3DEN.

This is a request for more information to be provided on how exactly the UIposition config entry is supposed to be used in conjunction with the aircraft picture so that the pylon combo boxes are positioned correctly when implementing one's own pylons interface.