anchor.diff:  === Automatic data tier migration
   
   {ilm-init} automatically transitions managed
  -indices through the available data tiers using the <<ilm-migrate, migrate>> action. 
  -By default, this action is automatically injected in every phase. 
  -You can explicitly specify the migrate action to override the default behavior, 
  +indices through the available data tiers using the <<ilm-migrate, migrate>> action.
  +By default, this action is automatically injected in every phase.
  +You can explicitly specify the migrate action to override the default behavior,
   or use the <<ilm-allocate, allocate action>> to manually specify allocation rules.
  
positive.code: 
  Data stored in the content tier is generally a collection of items such as a product catalog or article archive.
  Unlike time series data, the value of the content remains relatively constant over time,
  so it doesn't make sense to move it to a tier with different performance characteristics as it ages. 
  Content data typically has long data retention requirements, and you want to be able to retrieve 
  items quickly regardless of how old they are. 
  
  Content tier nodes are usually optimized for query performance--they prioritize processing power over IO throughput
  so they can process complex searches and aggregations and return results quickly.
  
negative.code: [[warm-tier]]
  === Warm tier
  
  Time series data can move to the warm tier once it is being queried less frequently
  than the recently-indexed data in the hot tier.
  The warm tier typically holds data from recent weeks.
  Updates are still allowed, but likely infrequent.
  Nodes in the warm tier generally don't need to be as fast as those in the hot tier.
  For resiliency, indices in the warm tier should be configured to use one or more replicas.
  
  [discrete]
  [[cold-tier]]
  === Cold tier
  
  Once data is no longer being updated, it can move from the warm tier to the cold tier where it
  stays for the rest of its life.
  The cold tier is still a responsive query tier, but data in the cold tier is not normally updated.
  As data transitions into the cold tier it can be compressed and shrunken.
  For resiliency, indices in the cold tier can rely on
  <<ilm-searchable-snapshot, searchable snapshots>>, eliminating the need for replicas.
  
  [discrete]
  [[data-tier-allocation]]
  === Data tier index allocation
  
  When you create an index, by default {es} sets
  <<tier-preference-allocation-filter, `index.routing.allocation.include._tier_preference`>>
  to `data_content` to automatically allocate the index shards to the content tier.
  
  When {es} creates an index as part of a <<data-streams, data stream>>,
  by default {es} sets
  <<tier-preference-allocation-filter, `index.routing.allocation.include._tier_preference`>>
  to `data_hot` to automatically allocate the index shards to the hot tier.
  
  You can override the automatic tier-based allocation by specifying
  <<shard-allocation-filtering, shard allocation filtering>>
  settings in the create index request or index template that matches the new index.
  
  You can also explicitly set `index.routing.allocation.include._tier_preference`
  to opt out of the default tier-based allocation.
  If you set the tier preference to `null`, {es} ignores the data tier roles during allocation.
  
  [discrete]
  