Hi @czemel
What is the difference and purpose between the Profile email address (also used to login) and the billing email address? How does Woocommerce use each one?
Most of the times these two fields (Profile and Billing) would have the same address on your site.
However, and depending on how the user decides to checkout this will be classified as a Customer or Guest.
By default, a user on your store can checkout as either a customer or a guest.
Customers are those that have an account on your store that they can login to. That means that either during checkout or from the My Account page, they opted to create an account and now have their details saved on your store for future purchases.
Guests on the other hand are those users that have made a purchase from your store but didn’t create an account. Of course, orders made by guests show up alongside orders made by customers in the Orders list and in the Orders (and similar) reports.
More information about both options can be configured within your WooCommerce settings in the “Accounts and Privacy” tab: https://woocommerce.com/document/configuring-woocommerce-settings/#accounts-and-privacy-settings
Hope that helps!