You can't reserve units ahead of time- it's on a walk-in basis at the wilderness access center the day before your first night in the backcountry.

Each unit has a quota (only 2 people, 4 people, 6 people etc) depending on the size of the unit.

So one's first night out is usually in one of the less desireable units since the choice units are usually already taken.

But you can use Igloo as your first night & then have a better chance to get a choice unit your second, third, 4th night.

If you go in by yourself as I do, you have a much better chance getting your top choices. A group of 2 is next best of course.