Low volume production of the tested devices means making 50-300 devices on full-blown assembly lines, injection molding machines and using the whole production line as if thousands of the devices would be created. This way we can test for production efficiency, and make sure that the third party tests are performed on devices that the end user will receive.
This is probably the most rewarding time throughout the development of an embedded system, since the product is ready for use, the tests performed at this phase are really just for reassurance, and to make sure that the assembly line performs as expected.