In Automation, your cars and engines are arranged using the model and family system. This system allows a lot of versatility and simplification of your vehicles and engines, as both your individual cars and engine will have bases upon which they can share production and engineering.
The cars you build are all sorted using the model system. In this system, the model refers to a group of various cars. All the cars within a model group have the same chassis. This means the cars within the model are built from the same materials have the same wheelbase length, chassis type and suspension geometry. This commonality between the cars allows for simplified production and engineering. Each individual car within the model is called a trim. Trims can be differentiated between each other based on aesthetics, equipped engine and drivetrain, or even body style. This entire system allows you to build one model with multiple purposes. For example, you can build a family of cars that appeal to different demographics (an affordable base trim, a luxurious trim, and a sporty trim), that are powered with different engines, equipped with different drivetrains and designed with different body styles (a hatchback, sedan and coupe variants).
The engines you build are all sorted using the family system. In this system, the family is the group of various engines. All engines within a family have the same basic architecture (cylinder count and layout, and valvetrain design). This commonality between the engines simplifies production and engineering. Each individual engine is a variant. Variants can be distinguished from one another based on displacement, tune, aspiration method, fuel systems, and exhaust systems. For example, you can build a range of inline-4 engines that fulfill different purposes, such as a cheap inline-4 engine, an economical smaller-displacement inline-4 engine, and a sporty, turbocharged inline-4 engine. For more information on building engines, visit the engine designer page.