What's up with Custom Data?
Knowledge Base Article
During your time working with Kentico, you may have stumbled upon a field whose name includes “CustomData.” These fields can be found in many of the E-commerce related objects, such as ShoppingCarts, Orders, and CalculationRequests. In some cases, such fields can come in handy, storing additional E-commerce information that the objects don’t already contain. Their utility is furthered by the fact that in many cases, CustomData fields are automatically transferred to related objects.