From the User Guide:
- Preserve color detail: this slider applies only to matrix based ICC profiles, so only display profiles – not printer profiles. Set at 50 by default, the slider lets you alter saturation and details in saturated portions. By reducing the value (to the left), the slider maintains saturation to the detriment of the details and, To the right, the details are preserved to the detriment of the vivacity of the colors. Note that the Export menu has a Preserve color detail checkbox which gives you the option to apply the slider’s effect to the exported images.
The help text in PhotoLab’s Soft Proofing palette isn’t as clear.