In some release circles, 'X' can also signify an "NTSC" (North American/Japanese) region version.
Check the file size. A standard Melee ISO is about , though compressed versions (like those marked with 'X') may be around 600 MB to 900 MB . If the .rar is only a few kilobytes or megabytes, it is likely a virus. sc24280-SSRBMX.rar
This is likely an internal tracking or release number used by specific online archival groups. In some release circles, 'X' can also signify