Consider a writer called Mr. K. He writes a parody of another writer’s work and publishes it online.
The second writer, let’s call him Mr. S, takes almost all of K’s parody and republishes in a book without permission, daring K to sue him for copyright infringement. K sells the book rights to his work to a third party, let’s call him Mr. 3, who jointly registers the copyright to K’s work with K and who does, in fact, sue S for infringement. S and 3 wind up settling the lawsuit, and S believes that he has gotten away with his infringement.
Emboldened, S registers the copyright for a second one of his books and threatens to file a lawsuit against a fourth party (Mr. G, who S believes to be K) for some imagined infringement of his second book. Because he waited almost four years after the date of first publication of his book, S is outside of the registration within three months of publication window for statuary damages. All he can claim is his actual damages from lost sales—on a book that hasn’t sold any copies for months.
K notices that S published a pdf copy online of the book containing the ripoff of his parody. Since his copyright was registered within the three month window of first publication, since S’s dare to sue opens the possibility of enhanced statuary damages of $150,000, and since K is not a party to the settlement agreement between S and 3, K realizes that he has standing to sue.