The "Build" number typically identifies a specific patch or update version archived on SteamDB for version control or distribution purposes. Files distributed in .rar format outside of official storefronts like Steam are often used for:
When downloading .rar files from third-party or unofficial sources, you should always before opening. Archives from unverified sources can occasionally contain malware or unwanted software. Safe Download Sources Forts.Build.10215710.rar
Frequently offers the game and its DLCs, often at a discount. The "Build" number typically identifies a specific patch