pts format: Explanation of i in the documentation missing

I'm working with Open3D with data from different devices. One of them does not provide the point cloud as a file, so I am creating the file by myself with a little Python script.

However, I am confused by the supported formats:
The **pts format** contains lines of `[x, y, z, i, r, g, b]`. xyz and rgb are intuitive and explained, but **what exactly is the i for?** And in what format is it expected?
I looked through the [documentation](http://www.open3d.org/docs/release/tutorial/Basic/file_io.html) of both Python and C++ and searched on the internet for the pts format, but found no easy explanation.