Pretty much, yes. Perhaps it helps to think of it this way: every node needs to be able to find the master node when it starts up, and the best way to ensure it can do so is to give the node a list of every master-eligible node, because this list must include the address of the elected master. You can give it more addresses and it'll still work, but it's a bit less efficient as it needs to check a lot more addresses before it finds the master. It's also a pain to keep such a list up-to-date as your cluster grows, and it's unnecessary because master-ineligible nodes are irrelevant to the process of finding the master.
On the master-eligible nodes themselves you technically don't need to include the node's own address in the discovery.seed_hosts list, but we generally recommend doing so anyway: it's simpler and less error-prone to have the same list on every node.
That setting should be a list of the addresses of the nodes you want to coordinate the indexing of the data coming from Logstash. All nodes have the ability to do this, but generally you want to avoid using the master nodes for anything involving actual data like this.
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