Restrict Content Pro version 3.0 added separate pages for "Customers" and "Memberships". If you've just upgraded from version 2.x you might be wondering what the difference is between these two.
Customer records are like user profiles. They contain information such as the member's name, original join date, email verification status, and last login date.
Memberships contain information about the membership itself, such as the status, expiration date, payment gateway that was used, etc.
Separating out customers and memberships was done in preparation for supporting multiple memberships per customer in version 3.1. The logic is similar to an e-commerce site that sells products, where you may have "customers" and "orders". Each user will only have one customer record, but each customer may have multiple orders. As of version 3.1, each user has one customer record, but it is possible for each customer to have multiple memberships (if enabled in Restrict > Settings > General).