anchor.diff:  
   This rule uses the Change Context and, like the cancellation rule above, specifies when a plan change should occur.
   
  -image:https://github.com/killbill/killbill-docs/raw/v3/userguide/assets/img/catalog/catalog_change_timing.png[width=960,align="center"]
  +image:https://github.com/killbill/killbill-docs/raw/v3/userguide/assets/img/catalog/catalog_change_timing.png[width=550,align="center"]
   
   There are three options (two of which are illustrated above):
   
  
positive.code: 
  === Plans
  
  image:https://github.com/killbill/killbill-docs/raw/v3/userguide/assets/img/catalog/catalog_plan.png[width=960,align="center"]
  
  Plans specify the terms of a subscription contract. Each Plan has a name and refers to the purchase of a single product. The details of a Plan are arranged by phase.
  In the illustration above we are showing a Plan with a 30 day Trial followed by an unlimited recurring or "evergreen" phase.
  
negative.code: 
  This rule uses the Phase Context and is used to specify when a cancellation should occur.
  
  image:https://github.com/killbill/killbill-docs/raw/v3/userguide/assets/img/catalog/catalog_cancellation_timing.png[width=550,align="center"]
  
  There are two options (illustrated above):
  
  