A more thorough explanation:

Definition: Marking estoppel is a legal principle that prevents a party from denying that a product is covered by a patent if that party has marked the product with a patent number.

Example: Company A manufactures a product and marks it with a patent number. Company B later claims that the product is not covered by the patent. However, because Company A marked the product with the patent number, they are estopped from denying that the product is covered by the patent.

This example illustrates how marking estoppel works. If a party marks a product with a patent number, they are essentially making a representation that the product is covered by the patent. If another party relies on that representation and later challenges it, the party that made the representation is estopped from denying it.