Ok, bad choice of words in my previous post (e.g. 1st, 2nd). I was referring to the ways you can use to set the store's cache size. Both allow the same thing, that is, settings a store's cache size (for the underlying BDB JE system).
The 'set policy "cacheSize=..."' allows setting the cache size from the admin CLI (java -jar kvstore.jar runadmin) -- more precisely, "set policy" is used to change the default policy parameters (which are default values used during Replication Node deployment), so you can basically use it to change the cache size value that will be used when deploying a Replication Node.
Still using the admin CLI, if you want to change the cache size for a specific Replication Node you can use the "plan change-repnode-params" command, or for all the Replication Nodes in a store you can use "plan change-all-repnode-params".
The 'cacheSize=...' is used to set the cache size from the Admin Console in the "Plans & Configuration" tab. You can change the cache size for a single Replication Node or for all Replication Nodes in the store.