Does this fit into map making assistance? I suppose that you don't open maps with WinMPQ when you get "unknown"s. Well maybe in some cases with sound files or when some protectors remove the listfile for smaller filesize... (dunno if this is done in common unprotectors)
Well, I can try to explain it anyways; I'll keep it basic, though, I'm lazy, you know
.mpq archives are encrypted. This encryption makes it impossible to get the name of a file in the archive, but the file can still be accessed. Now most of the time (and usually in all map files) there is a file called "(listfile)", which keeps track of the files in the archive. WinMPQ automatically searches for this file (at least I guess so), and can identify some of the archived files that way. If there is no list file, the only way to get out the filenames is bruteforcing.