_primary The operation will go and be executed only on the primary shards.
_primary_first The operation will go and be executed on the primary shard, and if not available (failover), will execute on other shards.
_local The operation will prefer to be executed on a local allocated shard if possible.
_only_node:xyz Restricts the search to execute only on a node with the provided node id (xyz in this case).
_prefer_node:xyz Prefers execution on the node with the provided node id (xyz in this case) if applicable.
_shards:2,3 Restricts the operation to the specified shards. (2 and 3 in this case). This preference can be combined with other preferences but it has to appear first: _shards:2,3;_primary
Custom (string) value A custom value will be used to guarantee that the same shards will be used for the same custom value. This can help with "jumping values" when hitting different shards in different refresh states. A sample value can be something like the web session id, or the user name.