Answered: Add layer to a single map?

OK, so another newbie question, apologies if this is just plain stupid, however I want to add layer to a single map (an Emissive map).  Is the correct workflow to add the layer and then turn it off in all other layers (click box on left and then shift-click it?).  I have made a right mess of the masks attempting to do this.

I am, in general, confused about how editing a mask in one map affects all the other maps.

Would someone kindly set me straight?