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:

Independent archiving of specific game states. Safety Warning

Another authorized retailer that tracks price history and sales. Forts Patches and Updates - SteamDB

Since its 2017 release, it has received several major updates and DLCs, including Tons of Guns , Moonshot , and High Seas . About Build 10215710

To ensure you have the most secure and up-to-date version of the game, it is recommended to use official platforms: