afni.nimh.nih.gov

I noticed that the default warp type for 3dAllineate when using align_epi_anat.py is '-warp aff' (affine / 12 parameters). I was wondering why this default was chosen over other options such as '-warp shr' (rigid-body / 6 parameters)? For instance, are there benefits to using the affine_general warp type since it acts as a sort-of simple field-map correction?

Thanks!
Zarrar