The priority for combining the PhysicsShape.friction properties when two shapes come into contact. If the priority of one shape is higher than the other shape then the higher priority PhysicsShape.SurfaceMaterial.frictionMixing will be used. If the priority of both shapes are the same then simply the higher enumeration value of PhysicsMaterialCombine2D from both shapes will be used. This is assigned to the current PhysicsShape.surfaceMaterial.

public ushort frictionPriority { get; set; }