What is the pop-up for?
The pop-up was designed to warn you if you are attempting to rename a customer account accidentally, letting you choose to either edit or enter a new customer account.
Here is an example of what the pop-up looks like:
What does clicking the name of an existing account mean?
This will edit the customer account, allowing you to make changes to the name of an account which will be saved when the order is completed.
What does clicking No, create a new account with the name " " mean?
This will clear all the customer information and let you input a new customer account that will be saved when the order is completed.