What is .sln.docstates file created by Visual Studio Productivity Power Tools?

Question

Here is what Microsoft have to say about it:

The .docstates file new in this release of the Pro Power Tools. It is used by the Document Tab Well extension to persist and hydrate the state of the floating tab wells. This information was being stored in the hidden .suo file but we had to move it to a separate file to fix a set of crashing bugs (timing issues). In the next release of the Power Tools we will hide this file by default.

http://visualstudiogallery.msdn.microsoft.com/d0d33361-18e2-46c0-8ff2-4adea1e34fef/ (Q&A tab, "Is .sln.docstates file created by Productivity Power Tools?" discussion, post by Chris Dias)

So effectively it is a supplement to the .suo (solution user options) file, and like the .suo file it should not be checked into version control.