Arbitrary Output Variables are user-definable variables which can contain any data calculated by the shader. This is useful for example when doing multipass rendering. Using AOV's you can render specular, diffuse, SSS or any other data you can think of directly into a file (similar to using 'rgba' to write an image with an alpha channel).
- Aqsis ??? and higher.
Examples can be found under the topic aov of the Guide section.