It contains a portion of the compressed binary data. The 7-Zip software (or compatible tools like WinRAR or PeaZip) uses the index provided in the .001 file to stitch these pieces back together into the original file.
“It is logical that all the file of the same TYPE to have the same extension and therefore illogical to have a different extension for every file in the series.” SourceForge · 15 years ago 003.7z
Depending on settings, the file may appear as filename.7z.003 or simply filename.003 . Discussion on SourceForge suggests that keeping the .7z extension before the number helps the OS recognize the file type more easily. Usage Review It contains a portion of the compressed binary data
“When 7z opens a 7z archive it would just check if it ends with '. 00x'... then it would know that it is a part of a split archive and could iterate true all split files.” SourceForge · 10 years ago Discussion on SourceForge suggests that keeping the
When a user creates a large compressed archive and chooses to split it into smaller pieces for easier sharing or storage, 7-Zip generates a sequence of files. The extension .003 indicates that this is the of a split archive. Key Characteristics