The window "Domain" displays a domain as a
wireframe, a point (u0, v0) in the domain, and
the u-constant and
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
given domain, the images of the point and the curves from the
"Domain" window, and the tangent vectors Xu(u0,v0)
and Xv(u0,v0) to the
surface at the point X(u0,v0).
Checking the "Show
tangent plane" checkbox will show the tangent plane to X(u,v) at the
point X(u0,v0). 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.