Edit: I have spent some time analysing what goes on - I find the code logic quite hard to follow. So I wrote it out in english:

We have two pieces of information - a 'secret' which is stored in each Moodle database and is used as a unique idenitifer of a site, even if the url changes - and the url of the registration.

Then, when creating or updating a a registration we go through these cases..

First case:
If the secret DOES exist and URL is changed (but doesn't match an existing registration):

1. Ask the admin if they have moved or copied the site (in a web form).
   1. If moved
      1. Update registration in db
   2. If copied
      1. Send request back to actual site to get them the secret regenerated

Second case:
If either:
a) The passed secret DOESNT exist, but URL DOES exist.. or
b) The passed secret DOES exist and the URL exists (but they are different) then..

1. Attempt to unregister existing registration with matching url, with some controls. The controls are:
   1. Email the registered admin, to get them to confirm the 'unregistration'
   2. Only try to send an email to the admin once per day (to prevent 'spamming' the admin)