Thanks, John.

In the above link:

"R321.3.2 Membrane penetrations. Membrane penetrations shall comply with Section R321.3.1. Where walls are required to have a minimum 1-hour fire-resistance rating, recessed light fixtures shall be so installed such that the required fire resistance will not be reduced..."

How is a "membrane" defined ... how is it different from a "wall" in R321.2: "fire-resistance-rated wall assemblies"?