When you adjust the volume slider in Roon and exclusive mode is enabled, the volume slider in Roon sends volume commands to the driver. If the signal path light is purple in Roon, we are sending bit-perfect data to the driver. Roon does not currently contain a software volume control implementation of its own.
What each driver does with those volume commands is up to the manufacturer. Ideally, the driver would tell the DAC to implement volume control in the analog domain, or in a part of the DAC’s signal processing path that doesn’t adversely affect resolution.
In the real world, not all drivers or DACs do the same thing. Some of them appear to lose bit-perfect behavior when the volume is adjusted, and some of them seem to maintain it. In some cases, this varies based on configuration done on the DAC (example: I have a DAC here that has a setting for “digital” vs “analog” volume. The digital volume is not bit-perfect, the analog one is).
Essentially, Roon is doing the best we can to give the right stuff to the driver: bit-perfect data + an out-of-band volume command, but – what the driver author and DAC manufacturer do with it further down stream can vary from situation to situation.