If my understanding is correct, the option FixedReference is there so that one can rotate the Directrix about its longitudinal axis, and to align the SweptArea’s x-axis to it. In this case one can conveniently reference the same cross section (SweptArea) to multiple differently oriented curves. Modifying the orientation of the SweptArea just because the referring curve is rotated, as you suggested, would be a work around but would also mean creating an extra cross section to each curve.

Referring both models in the picture below, the FixedReference (0,1,0) is constantly perpendicular to the Directrix curve and results in the solid at top. That of (0,0,1) is also perpendicular and gives the one in the middle. If the above two are perpendicular, (0,1,1) must also be perpendicular, and should result in a solid at the bottom. All axes have a ‘LocalOrigin’.

On a another note:

The above coordinates of the FixedReference imply that ggRhinoIFC lays the ‘SweptArea’ on the y-z plane of the ‘object coordinate system’ and x-axis corresponds to the normal or ‘Axis3’ (hence it is 0 in all three cases). However, in the ‘EXAMPLE’ note of the linked explanation, it is stated that ‘Axis3’ is the z-axis or normal and ‘Axis1’ is the x-axis (FixedReference) which aligns with the local x-axis of the SweptArea. You might want to check this too.

The only problem here is the incomplete sweep of shown in the third picture of the first model, which is already correctly started.

Looking forward for your opnion on this.

Thanks,

Dawit