This information is part of the Modelica Standard Library maintained by the Modelica Association.
This blocks computes the output y as the tangent-inverse of the input u1 divided by input u2:
y = atan2( u1, u2 );
u1 and u2 shall not be zero at the same time instant. Atan2 uses the sign of u1 and u2 in order to construct the solution in the range -180 deg ≤ y ≤ 180 deg, whereas block Atan gives a solution in the range -90 deg ≤ y ≤ 90 deg.