anchor.diff:  `shard`::
   The name of the shard to which the policy rule should apply. If omitted, the rule is applied for all shards in the collection. It supports a special value `#EACH` which means that the rule is applied for each shard in the collection.
   
  +`type`::
  +The type of the replica to which the policy rule should apply. If omitted, the rule is applied for all replica types of this collection/shard. The allowed values are `NRT`, `TLOG` and `PULL`
  +
   `replica`::
   The number of replicas that must exist to satisfy the rule. This must be a positive integer. This is a required attribute.
   
  
positive.code: 
  [source,json]
  {"replica": 0, "freedisk": "<500", "strict" : false}
  
  [[collection-specific-policy]]
  == Defining Collection-Specific Policies
  
negative.code: 
  `sysprop.<system_property_name>`::
  Any arbitrary system property set on the node on startup.
  
  `metrics:<full-path-to-the metric>` ::
  Any arbitrary metric. eg: `metrics:solr.node:CONTAINER.fs.totalSpace`. Refer to the `key` parameter in <<metrics-reporting.adoc, Metrics API>>
  
  === Policy Operators
  
  