Hi @TimF Thanks!
Yes, if I don't check the Constrained to Plane box, I can specify the plane.
Sometimes when I try to use Constrained to Plane to modify an exisiting spline, my first reaction is to specify a plane, but NX asks for a CSYS. Yes, NX would try to derive the plane from the CSYS. But at this time I need to derive the CSYS from my desired plane first in my brain, and then let NX derive the plane back from my input CSYS. The process seems to be a little odd.