Prior to 4.2 it was recommended to use the forwarder license, however this is no longer the case.
Search head license (for distributed search)
A search head is a Splunk instance that distributes searches to other Splunk indexers. Search heads don't usually index any data locally, but you will still want to use a license to restrict access to them.
Note: In versions prior to 4.2, Splunk suggested using a forwarder license on your search heads, but now recommends that you add search heads to an Enterprise license pool even if they are not expected to index any data. If your existing search head has a pre-4.2 forwarder license installed, the forwarder license will not be read after you upgrade.
See also Types Of Licenses