Industry expert Tony O'Lenick explains the difference between a process claim, a compound claim and a product-by-process claim in reference to patents.
A process claim is exactly what it says. It is a patent directed to a process. An example would be, "a process for conditioning hair which comprises contacting the hair with an effective conditioning concentration of..." It is directed to and prevents one from practicing a specific series of steps in the process as defined in the claim.
A compound claim is directed toward a chemical. It has a structure and an example would be "a polyamide conforming to the following structure..."
A product-by-process claim defines a compound in terms of the method (manipulative steps) used to manufacture that compound. It is like the compound claim, as it covers the compound, but it describes the compound in terms of what steps are conducted to make that compound. An example of a product-by-process claim could be "a polyamide made by the amidification reaction of a poly fatty acid and a polyamine conforming to the following structures..."
The product-by-process claim reads like a process claim but is directed to a compound made by that process. This type of language is valuable in the case or polymeric structures where the specific structure of the product is somewhat ambiguous.