I think the default behavior (no flags) is to NOT copy the file if version is same or higher on disk (should show version conflict dialog saying: your file is older) 8 flag will force the copy of the file even if version is higher, will copy sp3res.dll to the folder with a temp name (you can specify but if you dont windows will generate a name) ????.tmp, then on reboot the file is replaced Edit: I think I may be wrong about 8 overwriting newer file, that is what 4 does So maybe 8 will also warn if file is newer on disk, it just waits to replace the file instead of doing it right that second