Plain Wine means an unmodified version of Wine that can be downloaded from this site or your distro's default package repository. Any versions of Wine (or their wineprefixes) that come with tools like PlayOnLinux are modified right from the start to work with whatever application you wanted to work. For example, it could install necessary extra libraries or override some internal Wine libraries. Since this kind of thing interferes with the error/debug messages and basically makes Wine work a bit differently than in a "normal" clean-prefix scenario, this is why you're asked to test on a plain version of Wine with a clean prefix. http://wiki.jswindle.com/index.php/Wine_Prefixes is an article on what a Wineprefix is, anyway.
