Summary of the differences between document.url and location.href of the current web page_javascript skills

The Fourth Edition of the Definitive Guide to JAVASCRIPT says that there is a difference between the two.
See document.URL section.
[document.URL is the accessed document URL, and window.location.href is the real document URL
I don’t understand.
Document.url may change after server redirection, but window.location.href always refers to the URL used to access the web page.
document.url can only be used as a read-only string