How to get disp field from General Registration (BRAINS)

Hi all,

I am confused by the output settings in General Registration (BRAINS). There are two different transform (Slicer Linear Transform, and Slicer BSpline Transform) apart from the output image volume that we can choose, but I wonder what’s the difference.
What I want is to get the image and the displacement field, intuitively I think the linear transform is the displacement field, but I am not sure.

Best regards,
Chuan