assignedEventHandler return a array of all assigned eventhandler.
For example, when you add a event handler to a player does it return the index of the event handler.

_EHkilledIdx = player addEventHandler ["killed", {_this exec "playerKilled.sqs"}];
(In this case is it _EHkilledIdx)

assignedEventHandler return a array of the index of all added event handler.

_return = player assignedEventHandler "killed";
_return == [0];

When we add 2 eventhandler for the "killed" event does it return [0,1]
When we add 3 eventhandler and delete the first one does it return [1,2]

assignedEventHandlerCode return a a string of the code from the assigned eventhandler.

_eh = _return select 0; //eg. 0,1 or 2 deeps how many EH's has been added
_return = player assignedEventHandlerCode ["killed",_eh];
_return = "{_this addScore -1}";

_return is the string of the code of the assigned event handler.

This command would be very helpful for hack detection.