The release notes for the 2023.2 Preview state, “The Lumen option is based on ray tracing and calculates one light bounce and one reflection bounce on surfaces, giving much better rendering quality results.”
That being the case, what is the Bounce Count slider for in the Lumen Reflection Settings? Moving it from 1 through to 8 doesn't appear to change the reflection behaviour. Eg. two mirrors forming a corner where 2 or more reflection bounces are required for a realistic result.
As a basic summary of this, the Bounce count affects how many times light will bounce off of a surface, carrying information about the previous surface to help light the next one. Setting the value to 8, for example, means the light will bounce 8 times, hitting 8 different surfaces and carrying lighting information about the original light source and the previous surfaces. This can help get better results in indoor areas, where the number of objects tends to be higher and more densely packed.
Another good way to see the effects this has is to place two mirrors opposite one another so that they face each other, but also capture some of the surrounding area. Increase the number of bounces and notice how the number of times the mirror gets recursively reflected increases before the mirror renders as a solid/opaque surface.
