It depends a little on the context, though I'm far from sure what ewx is getting at.

Consider "Lightbulbs are approximately spheroid things that emit light. The sun is a lightbulb." as compared with "The sun is a huge glowing ball that provides us with illumination. The sun is a lightbulb."

In the first case, "The sun is a lightbulb." is taken as shorthand for "The sun is an example of a lightbulb."