Update: I found an option for Link Relationship(XFN) in the Menu advance property. Megamenu is the only ‘Page’ in the menu that is using this feature. I tested the same number found in the Megamenu and created my own Megamenu.
I found the number used in the Link relationship (xfn) on the megamenu page in edit mode, in the address window. It appears to be the number of order that page was created/published(?)
I am sure you may want to add simpler instructions or screenshots…?