Polysurface environments allow you to restrict the particle’s position to a series of joined surfaces, or breps. Unlike surface environments, which map the agent’s position to a 2d plane representing the bounds of the surface, polysurface environments act in 3D space, so the particle’s position is really that 3 dimensional point you see. This means that forces such as path follow and attract will work as expected.
When the polysurface environment is created, it also computes the border walls of the polysurface, which are simply any naked edges, extruded along the normals of the surfaces. This allows agents to know which edges to avoid when the contain force is used.