@ScrapCodes thanks for commenting. We should definitely add some inline comments here. I checked with @pwendell yesterday about this and he told me there was a discussion, but he didn't remember the reasons anymore. Having the inline comment here ensures that wouldn't happen again in the future :)

The comment should focus on the use of these two in Spark itself (e.g. in what code path do we use ActorRef vs ActorSelection). No need to copy the Akka doc here, although it might be nice to also document the less-obvious points (i.e. the subtle difference you mentioned).