When a file is compressed into a multi-part archive, the software (like WinRAR) splits the data across several volumes. These are usually named in a sequence like: filename.part1.rar filename.part2.rar filename.part3.rar
The file extension signifies that the file is the second volume of a split (multi-part) archive created using RAR compression. This method is typically used to break down very large files—such as games, software installers, or high-definition videos—into smaller, more manageable segments for easier sharing or storage. How Multi-Part RAR Archives Work
How to Prevent Issues When Extracting Multi-Part RAR Archives