This dialog lets you create a closed CV curve when you click to create a CV in the same location as the curve's first CV.
Create panel > (Shapes) > CV Curve button > In viewports, draw a CV curve and click to create a CV in the same location as the first CV in the curve.
Modify panel > Select a NURBS object. > NURBS toolbox > (Create CV Curve) > In viewports, draw a CV curve sub-object and click to create a CV in the same location as the first CV in the curve.
Interface
Yes
Closes the curve and ends curve creation.
No
Keeps the curve open and does not end curve creation.