Soap fault assertion – A soap request has 4 parts .

1. Envelope
2. Header
3. Body
4. Fault( In some rare cases we have fault).

So we can add Soap fault assertion to make sure that fault is always present. If it is not present it will fail. So as soap fault is rarely present , in most cases this will fail.So if we add a Soap fault assertion and we execute our request we will get an error that says "Response is not a SOAP Fault"