Certain kinds of claims require specific types of allegations in the statement of defence failing which the defendant may face an argument that facts not denied are admitted or, if that is precluded by the rules of court, that facts which form the basis of a particular defence but not alleged in the statement of defence, cannot be raised at trial.
Where the defendant has an independent cause of action against the plaintiff, that is not normally raised in the statement of defence but in a separate or attached document called a counterclaim.