This information is part of the Modelica Standard Library maintained by the Modelica Association.
Four quadrant inverse tangent
Returns y = atan2(u1,u2) such that tan(y) = u1/u2 and y is in the range -pi < y ≤ pi. u2 may be zero, provided u1 is not zero. Usually u1, u2 is provided in such a form that u1 = sin(y) and u2 = cos(y). Arguments u1 and u2 need to be Integer or Real expressions.
The atan2 function can also be accessed as Modelica.Math.atan2.
atan2(1,0) = 1.5707963267949