Why can't I select a patch plane for extrude which converted from t-spline plane

[iSolid]

Hi~
I made a t-spline plane and converted to a patch plane.
But I can't select it for extrude.
Why? isn't it a plane?
What's more surprising is that opposite plane permit extrude after thicken or offset plane.
Somebody please explain this question.
Thank you!

[jeff_strater]

You are correct.  When a TSpline is converted to a BRep, the result is always a BSpline surface.  It may be a very planar BSpline, but it is not a plane, which is why you cannot sketch on it, or use it as a profile in Extrude/Revolve/Sweep (you can, however, use a BSpline as a profile in Loft).

What are you trying to achieve with this workflow?  If you want a rectangular planar patch surface, I'd just draw a rectangle and use the Boundary Patch feature.

[iSolid]

I know that can make a "planar" patch surface with patch feature.
I just wonder why they are different surface from "patch workspace" and from "t-spline".
One is a plane and the other looks like a plane, but is not.
I don't understand... I think I need to study more...
Thank you~