EM HEAT

it eis essentialy for emergency heat,
whatever way your wired your secondary heat source becomes your "emergency" heat.
It could not be economy, since its only called upon when your primary heat cannot handle the load. and thats only at start up. once your within a predeterminbed temp set, your secondary heat should drop out
sometimes they wire secondary, in direct conflict with primary as to not aloow the system to use both at the same time

It stands for Emergency Heat. (Back up heat) . Normally you will find it on heat pumps. It's used manually when the compressor goes out on a heat pump. It's used automatically by the unit when it's so cold outside the heat pump can't keep up. This happens when there is not enough heat in the air to draw heat from. This happens at around 23 degrees. I hope this explains what emergency heat is and what it's used for.