paramete.spad line 110 [edit on github]
ParametricSurface is used for plotting parametric surfaces in affine 3-space.
coordinate(s, i) returns a coordinate function of s using 1-based indexing according to i. This indicates what the function for the coordinate component, i, of the surface is.
surface(c1, c2, c3) creates a surface from 3 parametric component functions c1, c2, and c3.