1. Create a Constraint Model.
2. Click New Type and enter the Name of your Product 'Car' > click Create to add the Car Product.
3. Add the Type Association > Create Association > Object Category = Product > Object to Map = 'Car' Product > Save.
4. Click New Type and enter the Name of your Product Classification 'Motorcycle' > click Create to add the Product Classifiation.
5. Add the Type Association > Create Association > Object Category = Product Classification > Object to Map = 'Motorcycle' Product Classification > Save.
6. Save the CML.
7. Add a virtual type Quote, and Save the CML.
8. Include the relations to the Car Product, Computer Classification, and your error message.
9. Save and Activate the CML.

@(virtual = true)
type Quote{

@(sourceContextNode = "SalesTransaction.SalesTransactionItem")
    relation car : Car[0..1];

@(sourceContextNode = "SalesTransaction.SalesTransactionItem")
    relation motorcyle : Motorcyle;

    string Msg = "Cannot combine Motorcyle Parts with Car Products";

    message(car[Car] > 0 && motorcyle[Motorycle] > 0, "{}", Msg, "Error");
    
}

type Car;

type Computer;

When a User adds a Product associated with the Motorcycle Product Classification, the error message is displayed on the Quote header, and the Quote cannot be ordered until the Motorcycle Product Classification is removed from the Quote.