This setting is only used by the cluster_routing action.

action: cluster_routing
description: "Apply routing rules to the entire cluster"
  value: ...
  setting: enable
  wait_for_completion: True
  max_wait: 300
  wait_interval: 10

The value of this setting must be either allocation or rebalance

There is no default value. This setting must be set by the user or an exception will be raised, and execution will halt.