An S09 file is the tenth part of a file that was split using ZipSplitter, a program used to split large files into smaller parts. It contains a segment of a larger file, such as an .MP4 or .ZIP file, that is one part of 10 or more other parts. S09 files can be restored, along with the other parts (.S00, .S01, .S02, etc.) by double-clicking the .EXE file included with the split file(s).

ZipSplitter can be used to split a large file into smaller, self-restoring files so that they can be more easily saved on portable storage devices or sent via email. For example, if your example.mp4 file is too large to send via email, you can Zip compress and split or just split the file into smaller file sizes. If you choose to split it into 3 equal parts, the MP4 file becomes example.mp4.s00, example.mp4.s01, example.mp4.s02.exe. ZipSplitter appends the last part with the .exe extension, which, when double-clicked, combines the split parts of the MP4 file and restores the file.