Are you referring to this checkbox in the material editor?
If so, as the tooltip for it says, that setting won’t have any relevance when using the engine’s deferred renderer, which is the project default. Not sure it would be useful for you even if you were/are using the forward renderer.
Re: your other questions, I’m afraid I’m at the limits of my knowledge on this subject. (I’m primarily a programmer.) Hopefully someone else with some rendering expertise will come along and provide you some helpful answers!