Reference:Cutaway Textures

Syntax:

difference | intersection {
  OBJECT_1_WITH_TEXTURES
  OBJECT_2_WITH_NO_TEXTURE
  cutaway_textures
  }

When using a CSG difference or intersection to cut away parts of an object, it is sometimes desirable to allow the object to retain its original texture. Generally, however, the texture of the surface that was used to do the cutting will be displayed.
Also, if the cutting object was not given a texture by the user, the default texture is assigned to it.

By using the cutaway_textures keyword in a CSG difference or intersection, you specify that you do not want the default texture on the intersected surface, but instead, the textures of the parent objects in the CSG should be used.
POV-Ray will determine which texture(s) to use by doing insidedness tests on the objects in the difference or intersection. If the intersection point is inside an object, that object's texture will be used (and evaluated at the interior point).
If the parent object is a CSG of objects with different textures, then the textures on overlapping parts will be averaged together.