A USB compliance test is a test program that objectively evaluates whether a product complies with the USB standard. By participating in a workshop hosted by the USB-IF or by passing a test at a USB certification contractor, you can receive "approval" as a USB product.
Not every product you ship has to go through the USB compliance test, but if you want your product to carry the USB logo, it has to pass the USB compliance test.
The test program should be performed on the final product (customer's equipment). Please note that although the CP210x is USB certified, using the CP210x does not exempt it from USB compliance testing.