This is a count of file-map window misses, i.e., times a file's mapping window had to be reloaded because the needed disk block mapping wasn't already cached. An administrator watches this counter to detect inefficient access patterns to large or heavily fragmented files, since frequent window turns add I/O overhead. For example, a high vmsFcpWindowTurn count for activity on a particular large file might prompt the administrator to check whether that file is badly fragmented on disk.