A vector field is a special type of space warp that crowd members use to move around irregular objects such as curved, concave surfaces. You can also apply it to particle motion with the Particle Flow Force operator.
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Create panel > 
(Space Warps) > Particles & Dynamics > Object Type rollout > Vector Field
The Vector Field gizmo, a box-shaped lattice, is placed and sized so that it surrounds the object to be avoided. The vectors are generated from the lattice intersections. These vectors are, by default, perpendicular to the surface of the object to which the field is applied; if necessary, you can smooth them out with a blending function. The crowd members move around the object by traveling perpendicular to the vectors.
You can use the vector field as a Space Warp behavior, as the source object for an Avoid behavior, or both. When used together, delegates slow down when they approach a complex object, and then go around it. This guarantees that delegates will not pass through the obstacle’s surface.
The vector field includes settings for strength, falloff, and a push/pull effect, as well as options to display the lattice, the effective range, and the vectors.