I mean the difference between he two processes

Dear Chinedu Onyeke ,

it is a matter of geometry...

For that you may have a look at page 12 of:

https://www-ssrl.slac.stanford.edu/conferences/workshops/xrs2016/documents/hgs_sxr2016_sxrd.pdf

The figure is self-explaining...

in short:

a) grazing incidence at angle alpha for all geometries; setting up a 'plane of incidence' in cooperation with the normal of the sample surface.

b) Reflectivity (XRR): incoming angle alpha; exit angle beta (= alpha for XRR)); in-plane/parallel with the plane of incidence.

c) Grazing incidence diffraction (GID): exit angle 2theta normal to the plane of incidence. 2theta range as in conventional XRD.

d) Grazing incidence small angle x-ray scattering (GISAXS): small exit angle gamma = '2theta' normal to the plane of incidence, but starting at very low scatter angles '2theta'.

Best regards

G.M.