(Here I eat my words on Meta about posting Unity questions on Unity Answers... since that site is less responsive than this one.)
Unity provides cell-based Occlusion Culling (via Umbra, I believe). However, a newer feature that it supports is Occlusion Portals.
The question is, if BSP-based occlusion culling is already a feature of Unity, what do portals add, and how?
PS. This question is not "What are portals?" -- I'm aware of the original Quake BSP-style portals -- which is partly why I find the explicit portal concept in Unity odd, since it uses BSP anyway.