Understanding differences between Instances, Part Properties and other SysML terms.

This video delves into the differences between Instances, Part Properties, Usage vs. Definition and other terms used in SysML. Cameo Systems Modeler is used with simulations to show the ideas in motion.
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[@Treff71]

Very informative and intuitive example - good job!

[@201mBrasil]

nice

[@p0rtabella]

So in this example, why wouldn't you just create the composition relationship between the "Car" and "Really Really Great Wheel" directly? If you know the specific type of part an element will have, why introduce this middle man? I understand generalizing to allow the types of wheels to inherit the correct properties, but I don't think I understand the abstract composition.