defaultGroup is only valid in outputs.conf, and is used to declare a group of indexers/search peers.
This can be either on forwarders, or cluster members (HF for internal logs).
It is not set/used in server.conf.
https://docs.splunk.com/Documentation/Splunk/latest/Admin/Outputsconf
[tcpout]
defaultGroup = <target_group>, <target_group>, ...
* A comma-separated list of one or more target group names, specified later
in [tcpout:<target_group>] stanzas.
* The forwarder sends all data to the specified groups.
* If you don't want to forward data automatically, don't set this setting.
* Can be overridden by an inputs.conf '_TCP_ROUTING' setting, which in turn
can be overridden by a props.conf or transforms.conf modifier.
* Starting with version 4.2, this setting is no longer required.
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