In the Splunk 7.1 Cluster Administration lab notes and in our environment, the cluster master appears as a SH in the Indexer Clustering: Master Node view of the Cluster Master. Why is it?
Interestingly, this view consists only of Peers, Indexes and Search Heads, which is also weird a bit.
If you look at document https://docs.splunk.com/Documentation/Splunk/7.2.4/Indexer/Systemrequirements#Additional_roles_for_t... , on Master Node you can setup Monitoring Console as well if Master Node is not overloaded and Monitoring Console runs searches only, so kind of we can say that Master Node can act as Search Head as well but splunk recommend that in Production environment do not use Cluster Master as Search Head. And looks like due to this Splunk only provides 3 views in Indexer Clustering Dashboard (Master goes under Search Heads view).
Note: The list includes the master node as one of the search heads. Although the master has search head capabilities, you should only use those capabilities for debugging purposes. The resources of the master must be dedicated to fulfilling its critical role of coordinating cluster activities. Under no circumstances should the master be employed as a production search head. Also, unlike a dedicated search head, the search head on the master cannot be configured for multi-cluster search; it can only search its own cluster.