what this case is made of
The barriers here do not repeat: it is wired into how the customer already works, it gets paid again without selling again, and it can hold its price and keep the customer. One is a reason to hold rather than a reason a rival cannot displace it, and it is not forced into a kind.
Read off the kinds of reason, not written by hand. No fifth held back and no summary above them that says something the four do not.