Reference: a unique identifier that is mandatory during the import of data. It is often, though not always, an id from an external data system. When importing data, the Reference should be unique within your whole Organisation, not just within the Event you are importing to. Any registrant records found with the same Reference will have the details updated – even if they are registered for a different Event. See Import Visitors for more details.
Registration Key: a unique number that is generated per registrant when either they register for an event or you import their details. If you choose to use the Login section of the Welcome page, the registrant can use this key to log in with.
Badge number: a unique number related to the registrants badge only. It is different to both the Reference and Registration Key. It is only of use to those exhibitors using Visit Connect.
Tag: used to easily identify a group of users. Tags can also be added during import; put the values in a column and assign that column to the Tag field during the import.