Companies (or individuals) you that purchase things from you. A customer has both a shipments and payments address, a gl account that payments should be applied against, and a payment term.
Stockor maintains a list of logins that belong to a customer.
If goods are not received immediately by the customer, a Sales Order is entered into the system to record the sale for future fulfillment, and an Invoice is generated when the goods are delivered (or shipped).
If the customer takes immediate possession of the items, then only an Invoice is generated. For instance the Point of Sale application only generates Invoices.