If there is the need to show information about virtual axes, which are not actively run by a power train, then these axes shall be provided, but they don´t have References to a power train. An example for a virtual axis is, when a robot control calculates the movement of an external axis in accordance to the robot movement, e.g. for a servo welding gun mounted at the robot flange, but doesn´t control actively the movement of this axis with an internal power train.
Another example for a virtual axis can be found in a delta robot. When the fourth axis is driven through a telescope shaft and cardan joints, then the length of the telescope shaft is depending on the positions of axes 1, 2 and 3. This length can be seen as a virtual axis, as it has constraints similar to a real axis, e.g. position limits. But it is not possible to actively move this axis.