It is a complex function of two complex variables.
A complex function of two variables is created through menu Calculus - New complex calculation - Complex function of two variables.
Such a function may be modified through icon .
For example,you can define a real function of two variables by choosing f for name, z and z' for the variables names and 2*Re(z^2)*z'+ 3*z + z' + 1 fro formula.
See also : Complex calculus syntax.