As I mentioned above, Timers should be automatically cancelled on deactivate, so you don’t have to worry about that.
Other “external” events could be anything you may have implemented in your own game. Basically, by external, I mean an event that is called by code or blueprints outside of the behavior tree (or at least outside of the subtree in which your node is executing). One way that could be done is by calling a function on some other blueprint (like the AI pawn) which registers for that pawn to call the event on your node when the event occurs. If it’s done by calling a “register” function, you’d need to call “unregister”. If it’s done by setting a variable or reference, then clearing that reference to none would be correct. However you are causing yourself to get the event called from outside the normal tree flow, you need to make sure you aren’t receiving that event when your node is not active.
Fortunately, we handle the most common built-in events for you (Timers, Timelines, and Delays), so no need to worry about those!
I hope that helps. 