These radiobuttons sets the current axes to either X, Y and Z . In 2D and Projection the projections are done on the current axes. In Projection the current axes decides along which axes the Axes slider should slide the slice plane.

With the Axes slider the slice plane can be moved along either the X, Y and Z axes. If the Axes slider is moved all the way to the left the slice plane of the current axes is removed. The slider is only enable for Slice .