XAudio2 filter radian frequency In the documentation for the [XAudio2 filter parameters] it mentions

Filter radian frequency calculated as (2*sin(pi*(desired filter cutoff frequency)/sampleRate))


Where does this formual come from? What is "Filter radian frequency" and "filter cutoff frequency" and what is the difference. Can someone translate this terminology to mathematics of DFT?
That is simply converting analog frequency into digital frequency. For example, if you want a cutoff of 1kHz at a sample rate of 44.1kHz, the corresponding digital frequency is 2*pi*1000/44100 = 0.142... radiants.
In terms of DFT the digital frequency is the $ \omega $ in the $e^{^-j\cdot \omega }$ term of the DFT