We actually pass this to JRoute so that they will apply the necessary Itemid on the registration links. I am not really sure how XIPT works but the registration link is actually linking through normal Joomla users. If you are using JomSocial, i believe their router.php should be adding the necessary item id's.
yes it works..but only if i add register link with Itemid! same for Forget Username and Password. if i use here J Route then i get two different com_users url for same Page:
If I recall correctly, JomSocial actually checks to see if there is any menu that is created based on the view. Hence, that is why you need to create the menu to ensure that the register link is getting the appropriate item id.