In order to compute spatial quantity (magnetic flux, current density, etc) on a surface, you have to use the integral method as follows:

Menu Advanced→Integral of a spatial quantity→Integral on face.

For instance in order to compute the magnetic flux on a surface you can either use integral on face(s) associated with a volume region, or integral on 2D grid.

NOTA: The integral method on the faces is more accurate as the computation is established directly on the node elements. With the integral method on 2D grid, the computation takes more time and the results are not as precise as the first method.