A more thorough explanation:

The lost-sales-of-unpatented-items theory is a legal concept used in patent law. It refers to a situation where a patent holder seeks compensation for sales of unpatented items that they would have sold along with their patented items, but for the defendant's infringement.

For example, let's say a company holds a patent for a new type of phone case. They also sell screen protectors that are not patented. If another company starts selling phone cases that infringe on the patent, the original company may argue that they have lost sales of their unpatented screen protectors because customers who would have bought both the phone case and screen protector from them are now buying the infringing phone case from the other company and getting a screen protector from them as well.

The lost-sales-of-unpatented-items theory is used to calculate damages in patent infringement cases. It allows patent holders to seek compensation not only for the sales they lost directly because of the infringement but also for the sales they would have made if the infringement had not occurred.