A more thorough explanation:

The sole-source rule is a principle in false-advertising cases that states a plaintiff cannot recover damages unless they can prove they have a monopoly on the sale of goods with the advertised trait. This is because only then is it clear that the plaintiff would be harmed by the defendant's advertising.

Let's say Company A claims in their advertising that their product is the only one on the market that can cure a certain illness. If Company B, who also sells a product claiming to cure the same illness, sues Company A for false advertising, they would not be able to recover damages under the sole-source rule unless they can prove that they have a monopoly on the sale of their product.

Another example could be a company claiming that their product is the only one made with a certain material. If another company sells a similar product made with the same material, they would not be able to sue for false advertising under the sole-source rule.

These examples illustrate how the sole-source rule protects companies from false advertising claims unless they have a monopoly on the sale of the advertised goods.