"First Choice Only" means that if you want to draw that tag then you must have it as your first choice, it cannot be drawn as a second choice. Most of these hunts now limit NR tag numbers. Previously if you chose one of these areas as your first choice it was a guaranteed draw so there would be no point to having a second choice.
You can still select a 2nd choice but you should understand that Idaho looks at everyone's First Choice before looking at anyone's Second Choice. Meaning that the second choice only matters if there are more tags available than first choice applicants for a given hunt. This almost never happens for antlered deer tags.
Unlimited tags are not offered during the second draw.