A crowd system is a way to handle a large number of characters at the same time in the same environment. They need to be able to move around and avoid each other. The agent tell the crowds system where it wish to go and it will get back a path and movement orders so it avoids running into other agents. For that the crowd system need access to the pathfinder search graph so it can see what parts are valid and can replan when paths collide.
Reference
Postmortem: Developing Systemic Crowd Events on Assassin's Creed Unity - 2015
RVO2 UDK - 2011
RVO2 Library - 2010
PLEdestrians - 2010
Simulating Crowd Dynamics: Flow Lanes and Character Animation - 2008
GPU-Based Scene Management for Rendering Large Crowds - 2008
GPU Crowd Simulation - 2008
Controlling Individual Agents in High-Density Crowd Simulation - 2007
Comparison Of Crowd Simulation For Building Evacuation And an Alternative Approach - 2007
Continuum Crowds - 2006