On one of my IDX sites, prospects can save searches if they register an email address on the IDX system.

There is also a little box for them to check if they want to receive auto-update emails with new listings daily so they don't have to manually return to see updates.

Hi Kelly.  An IDX registration is when someone goes onto your site and they register by entering their email and any other info you required (such as name and phone) before going onto your IDX link to search properties.  A listing notification, I believe, is when that person actually enrolls on your site to be notified when a property meeting their search criteria becomes available.  Hope that helps.

IDX=Your MLS

Listing Notifier=Handshake listings