Plained heads just means that the heads have been surfaced (to make sure the surface is flat that goes onto the block) it also makes cumbustion chamber smaller. whatch for heads that have been shaved too much you will have to mill intake so it will fit
...... Mill... Plane...Surface.... basicly all mean the same.. Machined flat to better mate with the block!! But you can "shave" .020 off , or "mill" .020 off or plane.... get my drift?? just different parts of the country saying the same thing...cool??