MGM 16016-3s -- BEC how many cells?

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I'm having trouble understanding the manual for the MGM 16016 regarding the built in BEC. Some have mega BEC+, but the 16016 says OPTO. What is OPTO and how many cells is ok to use with it?

Is the switch that comes wired to the controller for BEC, or is it for the whole controller? i.e. if switched off is current being drawn from the batteries?

Mike -- it might be nice to mention that no wire or connections come with the controller, as now I have to order more wire. Luckily I ordered two sets of connectors to have extra, which is just enough to wire it up.
Thanks!
--Jonathan