Rigid Axis is like Wall, but it rotates around its center-point. It is often used to make a hammer or other rotating device by putting Jet inside one end of it. This makes it spin wildly.
Axis can exist in a liquid state, either by pressing Shift+X to pour some, or by mixing it with Inflow. Liquid Axis does not seem to have any special properties.
The material XOR is useful for cutting perfect circles out of a section of Wall.
Axis can be moved around when mixed with User. Axis only seems to work with Rigid.