The secondary uv texture coordinate at the impact point.

This can be used for 3D texture painting or drawing bullet marks. If the collider is not a mesh collider, Vector2.zero will be returned. If the mesh contains no secondary uv set, the uv of the primary uv set will be returned.

	// Attach this script to a camera and it will paint black pixels in 3D 
	// on whatever the user clicks. Make sure the mesh you want to paint 
	// on has a mesh collider attached.
	function Update () {
	    // Only when we press the mouse
	    if (!Input.GetMouseButton (0))
	        return;
	    // Only if we hit something, do we continue
	    var hit : RaycastHit;
	    if (!Physics.Raycast (camera.ScreenPointToRay(Input.mousePosition), hit))
	        return;
	    // Just in case, also make sure the collider also has a renderer
	    // material and texture. Also we should ignore primitive colliders.
	    var renderer : Renderer = hit.collider.renderer;
	   	var meshCollider = hit.collider as MeshCollider;
	    if (renderer == null || renderer.sharedMaterial == null ||
	        renderer.sharedMaterial.mainTexture == null || meshCollider == null)
	        return;
	 
	    // Now draw a pixel where we hit the object
	    var tex : Texture2D = renderer.material.mainTexture;
	    var pixelUV = hit.textureCoord2;
	    pixelUV.x *= tex.width;
	    pixelUV.y *= tex.height;
	    
	    tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
	 
	    tex.Apply();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        if (!Input.GetMouseButton(0))
            return;
        
        RaycastHit hit;
        if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        Renderer renderer = hit.collider.renderer;
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (renderer == null || renderer.sharedMaterial == null || renderer.sharedMaterial.mainTexture == null || meshCollider == null)
            return;
        
        Texture2D tex = renderer.material.mainTexture;
        Vector2 pixelUV = hit.textureCoord2;
        pixelUV.x *= tex.width;
        pixelUV.y *= tex.height;
        tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
        tex.Apply();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		if not Input.GetMouseButton(0):
			return
		hit as RaycastHit
		if not Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), ):
			return
		renderer as Renderer = hit.collider.renderer
		meshCollider as MeshCollider = (hit.collider as MeshCollider)
		if (((renderer == null) or (renderer.sharedMaterial == null)) or (renderer.sharedMaterial.mainTexture == null)) or (meshCollider == null):
			return
		tex as Texture2D = renderer.material.mainTexture
		pixelUV as Vector2 = hit.textureCoord2
		pixelUV.x *= tex.width
		pixelUV.y *= tex.height
		tex.SetPixel(pixelUV.x, pixelUV.y, Color.black)
		tex.Apply()