I guess they must mean they work in the ultrasonic range. I think most people who buy these have tube gear that is very sensitive to ultrasonic vibration. I did a recent experiment with ultrasonic DSD filters. With single rate DSD the noise rises sharply right at 20K. Normally DAC's will have aggressive low pass filters just past 20k to filter out the sharp ultrasonics from single rate DSD. Our DAC is designed for 11.2Mhz DSD only. Because of this the low pass filter is set at a very high 180Khz. We use software based SRC/SDM to upsample all audio to 11.2Mhz. As default the software based upsampler has a low pass filter set @ 50K for single rate DSD. But I found with our DAC and both my headphones, and speaker system the sound was much better if I changed that filter point to 150K. So I was using it this way with all of my gear. Then 1 day I brought the system over to a local client's place that has very expensive tube amps. When we played single rate DSD, the noise floor was horrible. When turned up loud the noise almost was louder than the music. So I set that filter back down down to 50K, and the noise went away. Well we still had the standard tube noise, just not the other noise. So I really think most of these tweaks are effective only with gear designed back in the industrial age. Because with properly designed modem gear, these tweaks don't seem to be as effective.