That's the beauty; you get to decide! There is a very substantial pool of existing correlation searches that you can enable and if your desired search doesn't exist, you can create it. I guess the bast answer is "correlated attacks" and by that I mean that various things happen in multiple systems close to eachother in time. That is really the power of Splunk/ES.
In essence, It can be taught via SPL and alerting to detect and respond to any threat you wish.
There isnt really any "most effective" as such, as it can do anything you want, and do it consistently well.