UR28M headphone out impedance??

I choosing which headphones to buy to fit best my Steinberg UR28M
And as we all know, the headphones impedance must be fit in a certain way with soundcard’s headphones outs impedance to get flat response and lound enough
BUT… i can’f find headphone out impedance value in ur28m specifications!
it’s so sad that Steinberg didn’t post that information, especially if consider SO MANY questions on this forum are dedicated to select headphones to specified audiointerface. Detailed specs could solve these questions without asking them on this forum…
in specs i find only this “35 mW+35 mW, @40 Ohm” but how to understand “@40 Ohm” - is it means the headphone output impedance itself or just impedance of plugged headphones during the measurment?

Maybe someone knows or measured or someone from Steinberg tech team can tell me the headphone’s outs impedance value?