The Start Date and First Billing Date are of much importance when it comes to the billing of legacy assets. You are not allowed to create and rate a "Usage Input" within the period of the asset start date and the first billing date, typically known as the legacy period. Since Billing supports only postpaid type of usage consumption, the Remaining Billable Amount is always $0.00. Other features for usage-based billing for legacy assets are on the same lines as usage-based billing for normal assets. For more insights, refer to Understanding Usage-based Billing.