§ 1458 Failure to answer - New matter in answer not conclusive.

This rule deals with mandamus cases, which are court orders telling a government official or agency to do something they're legally required to do. It explains what happens depending on whether the defendant responds to the case.

If the defendant doesn't file an answer at all, the court will automatically grant the mandamus order. This means the person or agency being sued must do what the court is asking, since they didn't show up to argue otherwise.

If the defendant does file an answer and raises new facts or arguments, the plaintiff isn't automatically stuck accepting them as true. The plaintiff still has the chance to fight back. They can argue that the defendant's answer isn't good enough legally, or they can bring in their own evidence to prove the defendant wrong, either by directly denying the claims or by showing other facts that cancel them out.

In short, this rule protects the plaintiff's right to challenge whatever the defendant says. Just because a defendant answers the case with new information doesn't mean the plaintiff has to accept it. They still get a fair chance to argue their side at trial.