It’s my understanding that “High Quality Mode” forces it into constant bit rate, which is the best to use. I believe the sample rate defaults to 44 or 48, depending on your master session rate, at least my mp3 exports do that. I find the mp3 export in Nuendo really good. It helps to use a mastering plugin like Izotope Ozone to dither down to 16-bit and monitor mp3 output quality in real time.