In the Answer, all defenses to the allegations of the complaint must be raised, and all facts essential to supporting the particular defense must be included. These are called Affirmative Defenses. If you do not raise a particular defense in your answer, you will be prohibited from using or raising it later on.

Natomiast:
A Demurrer is used to tell the court that the allegations in the complaint do not provide legally sufficient reason for the defendant to be sued. A Demurrer questions only the legal sufficiency of the allegations, not their truth or the plaintiff's ability to prove them. In the Demurrer, the defendant must state what was left out of the complaint to make it legally insufficient.
Co nie wyczerpuje definicji zarzutów przeciwko żądaniom pozwu, gdyż stanowi wyłącznie zarzut formalny.