Events are a little different to other contexts because they are instantaneous whereas other contexts usually have a duration.
This means that it is nonsensical to specify that e.g. the screen brightness should be set to X for the duration of the event, so Tasker assumes that all settings actions should persist beyond the event.
For more information about specifying events, see the Event Edit screen.
The parameters are passed in the array %evtprm.
The order of elements of the array have values which match the order of the parameters of the event.
Example: if an event's second parameter is an Application, %evtprm2 in the launched task will be set to the label of the application which triggered the event.