You should use the Unicode features of the ZipArchive Library when the filenames, comments or passwords in the archives you use contain non-ASCII characters.
Without the Unicode support, the strings in archives are stored under Windows using the following code page:
filenames - the current system OEM code page (CP_OEMCP),
comments, passwords - current system ANSI code page (CP_ACP).
Under other platforms, all strings are stored using the current system's code page.
To use the Unicode functionality under Windows, you should compile the library and your application for Unicode. Under systems that use Unicode UTF-8 as the default code page (like Linux and OS X) there are no special considerations needed. On other systems, the Unicode support is not available.
Obviously, the programmers of Microsoft have not read these recommendations.