A ProductUsage might be<br/>        • Charged by one or several UsageVolumeBalanceDebit that decrements the corresponding balance<br/>        • And might be rated by one or several ProductOfferingPrices that will be charged to an invoice.<br/>A UsageVolumeProduct is an installed UsageVolumeProductSpec.<br/>Each UsageVolumeProduct has at least one balance (UsageVolumeBalance) valid at a given point. A UsageVolumeBalance is initialized when a UsageVolumeChargingRule applies a credit for a new valid period.<br/>Each balance might be decremented by one or several UsageVolumeBalanceDebits and credited by one or several UsageVolumeBalanceCredits.<br/>A UsageVolumeBalanceDebit results from the charging of a ProductUsage.<br/>A ProductUsageCredit results from the application of the rules specified on the UsageVolumeProductSpec through its characteristics (frequency of the credit, quantity of the credit).<br/>