Log Event action is the main App Event from the Facebook Documentation, with both the
valueToSum params. The action Add Log Data Parameter, as it says, holds the optional values included with your app event, hence the
parameters of the app event.
It also describes it from the action descriptions.
This is from the optional parameter action. You can include all pre-defined Add Log Data Parameter actions to the next Log Event action.
This would be sample of a complete implementation of a custom log event.
Also, it's worth noting, as dop2000 said, there are also automated events, which are logged after a successful login.
And these are the Facebook Analytics actions.
Feel free to check the documentation too: constructcollection.com/construct-mobile-facebook
I hope I was of help. Have a nice day.