anchor.diff:  sections, each usage section is billed independently: As a result, we may see one or several `USAGE` invoice items per invoice,
   each associated with a given subscription, usage section and billing period.
   
  -So regarless of the usage type the following basic fields will always be seen:
  +Regarless of the usage type the following basic fields will always be seen:
   
   * `planName`: The name of the `Plan` in the catalog
   * `phaseName`: The name of the `PlanPhase` in the catalog
  
positive.code: invoice item -- and the aggregate amount is computed by summing the sum amount. In order to activate this mode, one needs to set
  the system property `org.killbill.invoice.item.result.behavior.mode` to `DETAIL`.
  
  Each of the `USAGE` item will contain the following fields:
  
  * `quantity`: Quantity consumed for the unit (type) and on the given tier
  * `rate`: We reused the `rate` field normally reserved for `RECURING` type to output the price of the tier
  
  Also, the `itemDetails` json field will provide some additional details:
  
  
negative.code: 
  Late Usage Data:
  
  If for some reason, there is some late usage reported in these past 2 periods, the system will include this delta. Depending on the type of usage and mode (e.g `AGGREGATE`) we will see one or several `USAGE` item for this period to reflect this additional charge. If the late usage is older than these past 2 periods, it is simply ignored.
  
  Missing Usage Data:
  
  