For me, the documentation was a top selling point which included code and curl commands with your api key, ready to be copy and pasted if you were logged in.
Yeah, if it's qualified with graphical design, then will fit under UI (usually). But the argument goes that a product is "designed" (i.e. a plan of how it works, looks, and interacts with other systems), and thus has a "design", encapsulating everything from purpose through to visual look and feel.