Some DACS, like the Audioquest DragonFly and exaSound E12, have a USB-based volume control which allows you to control the unit's volume using the system level and the "slider" in whatever app used when playing music. I think this is a pretty rare option in USB DACs.
I think you mixing it up.
With USB you control yet again what is going into DAC, not output volume actual DAC. And it is not rare, but common feature of al USB DACs. You do not control (in this case) DAC volume, just volume of what is going into DAC.
Simple example of analog world -
Imagine thsat you have a preamp plugged into integrated amp (regular input). Then you can control volume of your speakers via frist preamp, without touching volume control of intergated. In our case the source (USB output) is that first preamp.