SVGMatrix::rotateFromVector

I'd like some clarification of the following snippet taken from
WD-SVG-20000303, Section 7.13, discussing the SVGMatrix interface:

"rotateFromVector 
Post-multiplies a rotation transformation on the current matrix and
returns the resulting matrix. The rotation angle is determined by taking
(+/-) atan(y/x). The direction of the vector (x,y) determines whether the
positive or negative angle value is used. "

The relationship between the vector direction and whether to use the +/-
derived rotation angle is not entirely clear.  Under what conditions is
the negative angle value to be used?

S.