A better attribute node is nice to one where you don't have to type things out

I've compiled a small list of uses on SE: http://blender.stackexchange.com/a/14267/599
I could adapt this for putting into the docs, but it'd be nice to have confirmation that these are all correct, and perhaps an idea what to do about the inputs mentioned in the code which appear to do nothing.

Is it accurate to say that this node is used as a sort of "testing node" for features which don't yet have their own node (volumetric textures for instance)?

@Sergey Sharybin (sergey) @Thomas Dinges (dingto) you can answer this best

Think we could just add "See a complete list of possible attributes here" and link to the SE answer.

even the SE Answer is missing some e.g. i think foam

@Aaron Carlisle (Blendify)

Are you sure foam is a thing?
I don't see any mention of it in the code..

I saw use it once which is why i started this report in the first place see https://www.youtube.com/watch?v=XI373g_vl6s

@Aaron Carlisle (Blendify)

After investigating a bit, it appears that this string is specified in the ocean modifier:

According to the tooltip this field specifies the name of a "vertex color layer", however it doesn't create or modify any vertex color layers visible in Properties > Object Data > Vertex Colors.
Is this a one-of-a-kind sort of thing? Or are there other "backend-only vertex colors" which can be specified in a similar fashion?

I think that's the only one I have heard of but if there is one case then there is likely more. I think all the cycles team is subscribed to this thread maybe the can answer

The ocean modifier dynamically generates a vertex color layer as it is executed. The ocean modifier is a bit odd here in that e.g. uv project and dynamic paint write to existing uv maps and vertex color layers, so it's less confusing.

I don't think it's necessary to provide a list of all attributes that are accessible, at this point the only good reasons to use the attribute node are vertex color layers and smoke attributes. The others are either internal attributes or things that are accessible through other nodes. I don't think the documentation should suggest using these.