.rar (Roshal Archive), specifically part 2 of a multi-volume set.

1.09, which often includes critical engine improvements or bug fixes in software like Arma 2 .

This file cannot be opened in isolation; you must have part1.rar and any subsequent parts to successfully extract the contents. Patch v1.09 – Arma 2 - Bohemia Interactive Community Wiki

If you are attempting to open this file, be aware of the following:

Files with cryptic names ending in .rar from unofficial sources are frequently flagged for malware analysis to ensure they do not contain trojans or unauthorized scripts.