This summary is not the rule. The rule text controls.
If someone sues you, you can bring your own claim against them in the same lawsuit, even if it has nothing to do with why they sued you in the first place. This is called a counterclaim, and it lets you settle unrelated disputes with the same person in one case instead of filing a separate lawsuit.
For example, if a business partner sues you over a contract dispute, you could also raise a completely separate issue, like money they owe you from an old personal loan, in that same case. The claims don't have to be connected. You just need to have your own claim against the person who sued you, and you can choose to include it in your response to their lawsuit.
This gives you flexibility. You are not required to bring these unrelated claims in the same case (that requirement only applies to claims connected to the same dispute). But if you want to resolve everything with that person at once, this rule lets you do it.
