Still climbing the electrics learning curve.

A few questions:
1.) When do you have to use an external BEC vs the one internal on an ESC. Is it a hard fast rule that when your supply batter is over 12v (from the "Everything you wanted ..." eBook) you have to use external?
2.) Are there any ESCs that have internal BECs that can handle the full voltage range that the ESC can?
3.) When and ESC says BEC - OPTO, what does that mean?
4.) Pros/Cons of an SBEC vs UBEC vs ?BEC

Thanks

-Jorden