From PhpCOIN Documentation
An invoice is a formal notice that a client owes you some money, and can be for one to dozens of products or services.
An invoice can be created manually by an administrator, or automatically when a surfer orders something from your website.
Invoices have a billing cycle such as monthly, quarterly, yearly, etc. and if marked as "recurring", when the next cycle is due phpCOIN will copy the existing invoice into a new invoice and automatically email the customer.
If a customer is overdue on a payment, phpCOIN can send a series of "nag" emails, with any or all emails sending an automatic cc to your support department (for account disabling, for instance).
Invoices can be marked as paid manually by an admin, or automatically when a customer clicks on a "PayLink". A PayLink could take the user to PayPal or a similar site for payment processing, or it could take them to a form on your site for collecting credit card info, or even just show them where to snail-mail a payment ~ whatever the site-owner needs. An invoice can have more than one payment applied to it
The admin parameter to disable invoices will remove all menu links and buttons that reference invoices, and if a surfer tries to access the invoices data directly via URL parameters they will receive a "disabled" notice.