An amended complaint is information added to an original complaint after its already been filed. As with other court rules on amendments, an amended complaint must be filed before a response is issued. Otherwise, it would not be fair or in line with court process. An amendment to a complaint can be any material fact that was excluded at the original filing date.
An amended complaint is usually a better developed claim than the original. It takes into consideration all of the factors that could possibly exist in a complainant's viewpoint. It usually clarifies an existing matter rather than adds another fact to it, but it can do either.