The window "Domain" displays a domain as a
wireframe, a point (u_{0}, v_{0}) in the domain, and
the u-constant and
v-constant curves
through that point in red and green, respectively.

The window "Surface X(u,v)" shows the cone X(u,v) = (av*cos(u),
bv*sin(u), cv) for
the
given domain, the images of the point and the curves from the
"Domain" window, and the tangent vectors X_{u}(u_{0},v_{0})
and X_{v}(u_{0},v_{0}) to the
surface at the point X(u_{0},v_{0}).

Checking the "Show
tangent plane" checkbox will show the tangent plane to X(u,v) at the
point X(u_{0},v_{0}). Note that the tangent plane is
not defined at the vertex (0,0,0) of the cone because that point is not
a regular point.