...

However, note that as with most of the caching systems, this may introduce bias, manifesting itself as blurred results due to interpolation from the cached values. Presence and opacity caching efficiency is driven by the opacitycachememory setting – a speed vs memory trade-off. The more memory is allocated to this cache, the more efficient the opacity reuse will be.

[Historical]

...

Bxdfs are allowed to compute both presence and opacity. In this case, when dealing with transmission rays, both presence and opacity are separately computed, and combined by the renderer into the transmission result across the surface.

[Historical] Presence vs. Bxdf Continuation

...