I think caliper is a fancy way of saying "diameter." If so, then it's really just a way to explain to stupid people that if you want to measure the diameter of a tree, you need to use a straight, rather than a curved measuring device. A caliper has arms that extend from either end of the ruler, allowing an exact measurement of the diameter. Of course, you could also use a flexible measuring tape and divide the circumference by pi, but again, stupid people don't know how to do this, at least not without a smart phone.