Custom attributes set by Create Copies from Polygon Mesh for its own internal use. You can also use these to control execution later in your tree or another tree.

In particular:

• CopyIndex is an integer per vertex that stores the index of the copy instance. All vertices in the same instance have the same index. For example, if there are three copies then the vertices of the first instance have a CopyIndex value of 0, the vertices of the second instance have a value of 1, and the vertices of the third instance have a value of 2.
• CopyIndex_PerPolygon is also an integer that stores the index of the copy instance but it's a per-polygon attribute.