Billing periods are a core object in generating invoices.
Billing period definitions set the dates on which a customer invoices, contain the group of subscriptions to invoice together, and a number of invoicing overrides. Billing periods define when invoices will be generated, the service dates of charges on those invoices, and what dates to prorate charges to.
Billing periods are created and reused as a subscription is created for a customer. It will reference the customer's billing options for when and how to bill, and the account's options, and then any pre-existing billing periods on that customer, depending on those options.
How they are defined is based on the options for when and how to bill. These options can be set at the account level, or overridden on a per customer basis. Some of these options will also check for pre-existing billing periods on a customer.
Billing period preferences can be set, such as the billing date, on the Settings>Financial>Default Billing Period Preferences page. Changing these options does not affect existing draft subscriptions, active subscriptions, or billing periods. Only completely new subscriptions will be affected. This includes not affecting existing draft subscriptions.
Subscriptions on the same billing period will be billed on the same invoice. On invoices, for subscription product related charges, the service dates are defined by these billing periods.
See Configuring Billing for more information.