Gridlinked USA

I just received a package from Tor US and tore it open expecting to find something sent to me for possible comment. Inside, two paperback copies of Gridlinked. They're fourth editions. Every time one of my books out there goes into a new edition they send me a couple of copies. What does this mean? It tells me nothing about sales. Gridlinked UK was up into the teens prior to the new covers, which was great, but how many books is an 'edition'? Well, it can range from a few thousands (sometimes even less) to the tens or hundreds of thousands, or maybe millions if you're J K Rowling. However, it does tell me that they've actually printed a new edition, which has got to be a plus!