FEC criterion and PDR were described in "Møller, H. et. al.: “Transfer characteristics of headphones measured on human ears” J.
Audio Eng. Soc., 1995, #43(4), 203-217". However, I do not have access to that and don't know the info first-hand.
What I've found was an article on designing an headphone specifically for binaural reproduction.
Actually, it was what I've found first and from that article I've got information on FEC criterion and PDR.
From what I seem to understand, FEC stands for "Free-air Equivalent Coupling" and PDR is "Pressure Difference Ratio". They are related to the fact that ear transfer function is dependend on acoustic impedance, which is changed by headphones. PDR is difference of frequency responce at eardrum in free air vs in headphones. FEC criterion is passed when PDR stays within some limit, however, I do not know what is that limit, as I haven't seen the original paper, but it seems it's within +- 10 dB, while headphones often have +-30 dB. PDR at +-0 dB means acoustic impedance equal to free air impedance across whole audio band. Impedance seems to be affected not only by pressurized chamber effect, but also by standing wave resonances. Impedance plot allows to estimate PDR plot and vice versa, to some extent.
The FEC criterion seems to be a very important factor for determining headphones ability to allow resolving the length of a sonic radius-vector from recording, i.e. depth of scene in all directions. Without passing the criterion, you could have ecxellent separation and positioning, but acoustically perceived distance from you to sound sources would be incorrect.
My main question is how to measure headphone's acoustic impedance? Preferrably in an more or less easy way, possible at home using only the headphone, electret mic and some baffle. Knowing the impedance plot one would be able to tune HP's acoustic impedance to allow for more realistic (and safer, without huge HF peaks) reproduction, particularly in scene departament.