I'm talking about freshman walk ons.
After you're a walkon your freshman year, if you're invited back the next year you will be invited to camp. You're still technically a walk on since you're not on schollie, but you've proven yourself and were invited to return.
My understanding is that NCAA rules state you can only have 85 scholarshipped athletes on the roster, so 20 in camp are "walkons", but almost always sophs or juniors.