If you just have a two-rail sweep without other external model components, you cannot make curvature continuous or tangent continuous.

This involves an issue about the continuity of curves and surfaces. The following will involve some advanced mathematics. I hope you have learned it and haven't forgotten it.

The so-called positional continuity, also known as G0 continuity, means that the surfaces or curves only coincide with the boundaries.

G1 continuity: tangent continuity means that the two not only coincide at the boundary, but also have the same derivatives, which is what we call tangent. We know that every curve and surface in NURBS modeling can be described by a function. The derivative here is the derivative of a function that represents a certain curve or surface

G2 continuous: curvature is continuous, and the two are in The second derivative of the mathematical expression at the boundary is the same.

And so on. Rhino can achieve G2 consecutive at most, while Alias ??can go even higher and achieve G4 consecutive. The higher the continuity level, the smoother the connection.

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So we see that continuity is a relationship between two curves or surfaces.

If you just make a sweep2 somewhere and nothing else is related to the generated surface, you will naturally not be given the curvature option.

If you already have a part of the model, such as two surfaces on a certain model that you need to connect, of course you can choose blendsrf, but if you want to define the cross-section of the blended surface yourself, in this case, you You can draw the interface curve and then use the edges of the two faces that need to be connected as Rails to perform sweep2. At this time, because the blended face you make needs to be related to the two connected faces, the continuity option will be given.