The resource cannot be found issue in asp.net

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C#

ASP.NET

Javascript

.NET

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I was going through an Asp.net project file.
In that they are navigating to a htm page when session got expired or session variable is null or empty like this

if (Session["UserID"] == null)
           {
               Response.Redirect("Expired.htm");
           }

in that htm. in onload function following piece of javascript is there

function SessionExp()
                {         
                    if(window.parent.length > 0) 
                    {
                        window.parent.location.replace(document.URLUnencoded);                   
                    }      
                 }

My doubt is that what is window.parent.length,document.URLUnencoded means and sometimes document.URLUnencoded reurns Nan, what is the meaning of that one?.
And if i open this website in two browser tabs and logout from one tab,and i clicked on any page on the second tab, instead of navigating to Expired.htm it is showing error message

> The resource cannot be found.

> Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.
Requested URL: /website1/folder1/undefined

how undefined came??
Note: Iframes are used.

Anyone Pls explain this....