There are two schemes in general use for specifying a rotation; one is in terms of its Euler angles, the other uses its axis and angle of rotation. In reality, there exists a fairly elaborate theory of the three dimensional rotation group, including a third parameterization in terms of quaternions which unifies the other two rather nicely.
The other way to represent rotations is to specify the angle of rotation and the axis about which it occurs. In many ways it gives the most direct representation of a rotation; an additional advantage is that to transform the rotation to a new coordinate system, it is only necessary to rotate the axis.
The user of GEOM needs to know little of this, other than to relate rotations to the positioning of the scene that is about to be drawn. For the programmer's part, it suffices to look up the matrix elements of the rotation matrix in some reference work. Nevertheless, without some understanding of the theory, it is difficult to get all the sign conventions and parameter ranges straight.