This setting is only used by the shrink action.

action: shrink
description: >-
  Shrink selected indices on the node with the most available space.
  Apply shard routing allocation to target indices.
  shrink_node: DETERMINISTIC
    allocation_type: include
    key: node_tag
    value: cold
  - filtertype: ...

The only permitted subkeys for post_allocation are the same options used by the allocation action: allocation_type, key, and value.

If present, these values will be use to apply shard routing allocation to the target index after shrinking.

There is no default value for post_allocation.