A product-by-process claim is a claim in which the product is defined by the method or process by which it is made. In chemical cases, such a claim is narrower than the traditional product claim because the claim only covers products that are made by the recited process.
For example, a steel casting that is made through a novel, nonobvious, useful process can be protected through a product-by-process claim. However, the claim would not cover identical steel castings that are made through conventional processes.
Please note that there is no general principle that would prevent an inventor from separately filing claims to cover the process or the product itself.
Here is an example:
1. A resistor which comprises: a ceramic core, a coating of carbon deposited on the core by decomposition of a hydrocarbon gas in the presence of the core, and a stripe of conductive metal.