If this archive is for a specific mod (like ProMods or a truck skin pack), the installation process may involve moving the extracted .scs files into your Documents/American Truck Simulator/mod folder. To help you further, could you tell me: Is this the base game or a specific map/truck mod ? Did you run into any errors while trying to extract it?

Ensure you have enough space for the extracted files (typically 7GB+ depending on DLCs included).

The file name indicates that this is the first segment of a three-part compressed archive.