Auto Repeat feature helps you create certain transaction automatically in a given time period.
Assuming that you follow deferred expense system for some items. It requires you to create same Journal Entry each month to Cr. Deferred Expense a/c to Dr. Expense Account. You can create first Journal Entry manually for it, and then create auto-repeat transaction for it.
To set the subscription for the sales invoice Go to Subscription > select base doctype "Sales Invoice" > select base docname "Invoice No" > Save
From Date and To Date: This defines contract period with the customer.
Repeat on Day: If frequency is set as Monthly, then it will be day of the month on which recurring invoice will be generated.
Notify By Email: If you want to notify the user about auto recurring invoice.
Print Format: Select a print format to define document view which should be emailed to customer.
Disabled: It will stop to make auto recurring documents against the subscription.
Difference between Subscription and Auto Repeat