A more thorough explanation:

A capacity defense is a type of defense used by a defendant in a legal case. It is based on the defendant's inability to be held accountable for an illegal act or the plaintiff's inability to prosecute a lawsuit. For example, if the plaintiff is a corporation that has lost its corporate charter, it may not have the capacity to sue. Another example is if the defendant is a minor, they may not have the capacity to be held accountable for certain actions.

Capacity defense is different from an affirmative defense, which is a defendant's assertion of facts and arguments that, if true, will defeat the plaintiff's or prosecution's claim, even if all the allegations in the complaint are true. Examples of affirmative defenses are duress (in a civil case) and insanity and self-defense (in a criminal case).

For instance, if a defendant is charged with assault, but they have a mental illness that prevents them from understanding the consequences of their actions, they may use an affirmative defense of insanity.