1.)
Enabled is a trigger that works perfectly.
Disabled is a trigger that will not work because either you chose to or there is something wrong with it.
Initially ON means that the trigger will be working in-game, everytime the event is met.
Initially OFF means that the trigger wont trigger when the event is met, and wont do nothing.
2.)
Turn on the trigger means that you will turn on a turned off trigger, and is usually used for enabling periodic loops, for example you have a trigger with
-
Events
-
Time - Every 0,03 seconds of the game
Having this Initially ON, is bad, because it requires a bunch of resources which can cause lagg, so you have it Initally OFF, and you turn it on, in another trigger, when needed.
Run the trigger, will do what it sais, it will Run the trigger (and its conditions if you want its optional) whenever you want to, ignoring if the event is met or not.
3.) Well, the Run Trigger function is used when a set of specific actions are required in more than one trigger, so instead of copy/pasting them you just put the function Run Trigger.
If you want to "load" a trigger, from that i understand you want to do something differently from what your main trigger does, with a different event, you will have to use Turn Trigger - ON, granted you have that trigger Initially OFF.
~Regards