Also figured out why some people have Stalker early while I have to wait until lvl 50 to get it! When you first go online to Create your soldier, you get a choice between 5 different classes. I unlocked Marksman or some crap. Anyways, depending on the perks you see on the classes you can choose, you unlock them in different orders. Not sure what class unlocks what when. But I'm sure someone will figure it out.