Anisotropic is an adjective and it applies to lots of things. It means "not the same in all directions". Everything in the Physical surface node is isotropic - the same in all directions, except the scatter, as hborre said. If you want anisotropic scatter, it's built in. If you want anisotropic specular reflections, as seen on hair or silk threads or grooves on a pan, you can't plug that into the Physical surface node. You'd need to do some other simpler/dumber root node, or you'd need to mix them.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)