.SII File Extension
18 Wheels of Steel: Haulin' Truck Definition File
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What is a SII file?
Data file used by 18 Wheels of Steel: Haulin', a truck driving game; contains properties for a truck, such as the engine class; saved in a plain text format and often altered for creating custom truck configurations.
SII files can also contain a list of referenced truck files for the game. For example, the file truck_storage.sii contains the list of truck definition files and can be extended to add new trucks. It is saved in a plaint text format.