A more thorough explanation:

Post-expiration-sales theory is a legal concept used in patent law. It refers to a theory of lost-profits remedy where compensation is sought for sales lost after a patent has expired. This is based on the idea that infringement gave the competitor a head start on entering the market.

For example, let's say a company holds a patent for a new type of phone case. The patent is valid for 20 years. After 20 years, the patent expires, and other companies can start making and selling similar phone cases. However, if a competitor started making and selling the same phone case before the patent expired, the original company could use the post-expiration-sales theory to seek compensation for the sales lost during that time.

Another example could be a pharmaceutical company that holds a patent for a new drug. Once the patent expires, other companies can start making and selling generic versions of the drug. However, if a competitor started making and selling the generic version before the patent expired, the original company could use the post-expiration-sales theory to seek compensation for the sales lost during that time.

In both examples, the post-expiration-sales theory is used to compensate the original patent holder for the sales lost due to the competitor's infringement. It is based on the idea that the competitor gained an unfair advantage by entering the market before the patent expired.