Documentation: Description of each layer

I have noticed that layer “Top Grab Areas” is needed to move top device, which is expected.

But I have also noticed that “Top References” layer should also visible which is not expected by me. So I took a look at the documentation to know what is exactly this layer, but I do not found anything about it.

So where is this information ?

And if you know, is it normal the need of the visibility of these two layers.