f(x,y,z)= A1 x + A2 x^2 + A3 x^3 + B1 y + B2 y^2 + C1 z + C2 z^2 + D x y
Each parameter sets the value of the coefficient of a different term.
In the Pisces user interface, the coefficients are
known by names such as
This is not a general cubic polynomial in three variables.