This summary is not the rule. The rule text controls.
If you're being sued and later get a new legal claim against the person suing you, or a claim you already had becomes something you can actually act on, you don't have to start a whole new lawsuit. Instead, you can ask the court for permission to add that claim to the case that's already going on.
This is useful because it saves time and keeps related issues together in one case instead of spreading them across multiple lawsuits. For example, if you filed your answer in March and then in June the other person breached a different agreement with you, or a debt they owed you finally came due, you could ask the judge to let you add that claim to the existing case.
The key thing to remember is that you need the court's permission first. You can't just add the claim on your own. You have to file what's called a supplemental pleading and explain to the judge why this new claim should be included in the current lawsuit.
