What does the black star mean?

The black star next to a customer's name is to alert you to the fact that it that customer's first booking.

The unique identifying code for each customer is their email address so if a new email address has been entered, the system will create a new customer record accordingly.

The black star either indicates a brand new customer has registered or it might be a returning customer who has used a different email address to the one in your database. If this is the case, you may want to move the booking to the original customer record and erase the new record to avoid duplication.