anchor.diff:  ==== Usage Invoice Items
   
   As described in the section http://docs.killbill.io/latest/userguide_subscription.html#components-catalog-usage[Usage Billing],
  -Kill Bill supports invoicing customers based on different usage type. The usage billing is always done on a per subscription
  +Kill Bill supports invoicing customers based on different usage types. The usage billing is always done on a per subscription
   level and for a given billing period; in addition, if the active `Plan` associated with the subscription contains multiple usage
   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.
   
  -Regarless of the usage type the following basic fields will always be seen:
  +Regardless 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: * `endDate`: The end date for the period being invoiced
  
  In addition, depending on the usage type for the usage section (e.g `CONSUMABLE`), and depending on the configuration of the system,
  we will see additional information. 
  
  ==== Capacity In Arrear Mode
  
  For each capacity in arrear usage section, we will see some json in the field `itemDetails`, to provide additional information:
  
  [source,bash]
  ----
  
negative.code: ----
  
  In this example we see that we were invoiced for 2 different units, and this shows the tier details for each unit, and the amount that will be charged.
  Because unit `members` reached tier 3, the amount will be based on the pricing for this tier.
  
  ==== Consumable In Arrear Mode
  
  This mode bills for each unit based on actual usage, using a price determined by the tier(s) reached and by the policy. With the `ALL_TIER` policy, usage within each tier is charged according to the price for that tier. With the `TOP_TIER` policy, all usage is charged according to the price for the highest tier reached. 
  
  We can configure the invoicing system to output one amount per tier (Detail Mode), or a single amount for all tiers (Aggregate Mode). The example below illustrates each mode for the `ALL_TIER` policy.
  
  ==== Aggregate Mode
  
  