From PhpCOIN Documentation
A bill is a formal notice that you owe a supplier some money, and can be for one to dozens of products or services.
A bill is created manually by an administrator.
Bills have a billing cycle such as monthly, quarterly, yearly, ont time, etc. and if marked as "recurring", when the next cycle is due phpCOIN will copy the existing bill into a new bill. This is handy for example to automatically track your server costs, since it would not normally change each month.
A bill can be manually marked as paid by an administrator, and can have more than one payment applied to it.
The admin parameter to disable bills will remove all menu links and buttons that reference bills.