Documentation -> Billing/Invoicing -> Customers

Customers in the flowlog Billing section refers to entities that pay you for services, products, etc. and the Customers feature is useful for keeping up with customer contact details, notes, etc. A Customer is a prerequisite for any Invoice. You can also view all the Invoices for a particular Customer from the main Customers page, which are organized by Unpaid and Paid statuses. This allows you to quickly see if there are any outstanding payments due from a particular Customer.