onCleanUp() vs onComplete() vs afterLaunch()

In Protractor, there is a "global set up" method called onPrepare(), but I'm not completely sure what is meant to be a "global tear down" - there are three relevant methods: onCleanUp, onComplete and afterLaunch that are all called after a test execution.

Why does protractor have three methods called after a test run? What is the difference between onCleanUp, onComplete and afterLaunch?

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I've also noticed that there is an "exit" event that we can attach a callback to (example here):

protractor.on('exit', function (status) {

});