This document shows you how to enable your guests to choose from a list of sponsors available to them. It uses the person being visited field to send the email for the user doing a self-registration to a person with privileges on approving guest accounts. This would be used in environments where you have designated a list of sponsors that can be used to approve accounts. Perhaps a few lobby ambassadors are responsible to create the accounts. This also helps the guest from having to enter in someone's email, remember it, etc.
The self-registration page settings require the person being visited to be enabled.