Unity 3.5: What is the definition of PLE algorithm?

Unity Roadmap 2011 → Next release: Unity 3.5 → Pathfinding states:

Unity will automatically generate navigation meshes from your level geometry.
Beautiful, natural-looking crowd simulation using RVO and PLE algorithms wrapped in a simple API. Agents can find paths to target locations with built-in crowd simulation, or can be moved directly on the NavMesh in a similar way to the character controller.

I was able to identify RVO as “Reciprocal Velocity Obstacles” but couldn’t find any PLE algorithm. I would greatly appreciate an answer including a source.
Thank you.

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