b_spline_surface_with_knots STEP

Hallo All!

While going through the STEP format as per the ISO 10303, for
b_spline_surface_with_knots, I was unable to decipher the cryptic
parameters. Being from a non-CAD background, I made the most of what I could
understand. Also taking help from the ISO, I was able to make out a few
parameters, for instance the degree, and the control points for the
surface, etc.

This is how the STEP file definition for the b_spline_surface_with_knots
looks like for a simple cube.

B_SPLINE_SURFACE_WITH_KNOTS(",1,1,((#61,#62),(#63,#64)),.UNSPECIFIED.,.U.,.U
..,.F.,(2,2),(2,2),(-2.E-1,1.02E1),(-1.02E1,2E-1),.UNSPECIFIED.);

The cube was made with dimensions 10x10x10. #61 to #64 are all cartesian
points

Now, I have not quite understood what the difference between the first Array
and the second array list is. Are they both Control Points? What do U, U, F
and UNSPECIFIED Irefer to in this case.

Will be thankful for any help that you can offer!

Regards
Barn