Unfortunately, it is not possible to manually classify a product in an A, B, or C category.
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However, it is possible to change the gross margin. For example, if you lower the gross margin of the C category, products will be more likely classified in the B category.

You can also have the ABC category calculated based on the number of sell order lines instead of gross margin.
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You can change this by going to the Settings and click on ABC Classification
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In the ABC settings, you'll be able to change the Category calculation.