The version numbers of PRTG Network Monitor indicate the year and the quarter of the release. So the version of PRTG which has been released in May 2012 is called PRTG 12.2, followed by the build number XXXX (e.g. PRTG 12.2.xxxx). After July first 2012 their builds carried the version number 12.3, and so on. This gives you a rough idea of how old a PRTG version is.