When setting up a Revenue Item for use in invoice line items throughout MC Trade, you can indicate a "number of days" for the default service period to use for a line item.
This is used to create the service period when manually creating an invoice in MC Trade, and/or when an invoice is created through Forms submitted online for purchase items using the revenue item.
Please note that generating invoices through the Auto-Generate tool will always use the Billing cycle to determine the service period, it will not refer to this Default Service Period field on the Revenue Item.