For instance when a 404 error is returned on the page.
SAM's HTTP User Experience Monitor will return a status of "Down" for any hard error, like 4xx and 5xx errors. Web Performance Monitor provides more granular visibility into web site or web application availability, including all content on the page. Below is a small example of that.
I don't have an http user experience in my list. hmm
They are listed there in the image you posted as the "HTTP Monitor" and "HTTPS Monitor"
OK, sorry was thinking it was a different one. Thanks,