Which optional configuration setting in inputs .

https://docs.splunk.com/Documentation/Splunk/7.0.3/Forwarding/Routeandfilterdatad#Perform_selective_indexing_and_forwarding

Specifies a comma-separated list of tcpout group names. Use this setting to selectively forward your data to specific indexers by specifying the tcpout groups that the forwarder should use when forwarding the data. Define the tcpout group names in the outputs.conf file in [tcpout:] stanzas. The groups present in defaultGroup in [tcpout] stanza in the outputs.conf file.