ADFS URL and Office 365

I have installed and configured ADFS 3.0 with O365 per article:

http://blogs.technet.com/b/rmilne/archive/2014/04/30/how-to-install-adfs-2012-r2-for-office-365-part-3.aspx

However I just want to know, how does Office 365 know the federation URL?  At which point during this configuration does that happen?  Because I have not configured the ADFS Federation URL anywhere throughout the documentation however when I tested SSO, it worked and I saw the redirection using federation URL.  

C