Difference between ('3D cut points datasets' + plot by the '3D cut points datasets') and (plot by the 'solution' + point selection)?

Hi I want to evaluate the displacement at a point on a shell of a 3D mode, first I use the a point graph and choose the datasets to the solution, then choose the point on selection and choose the 'shell.disp' as the value, this way worked. But I tired the second way, fist add a 3D cut point datasets, then I use a point graph and choose the datasets to the 3D cut point, then I can't find the 'shell.disp' expression.

I don't know whats the difference between the two ways.

Thanks