I think talking about coherent scattering combined with the interference phenomenon of x-rays and XRD is quite the same because the deflection of the x-rays at the atoms/lattice points in XRD are due to coherent scatter (also called elastic scatter).  May the points of view on how the 'diffraction data' are gatherd are different. I admit that due to the different measurement techniques (wave length dispersive versus energy dispersive measurement) the quality of the 'diffraction data' is very much different for example the peak resolution and from this the applicability of any refinement techniques.

For difference in crystallite size and grain size please see the link.

I share the opinion of the link: a grain consists of a lot of crystallites (if the grain itself is not a single crystal one).

But there are a lot of discussions on grains and crystallites on researchgate; see for example the links.

https://en.wikipedia.org/wiki/Grain_size

https://www.researchgate.net/post/How_accurate_is_the_measurement_of_grain_size_using_XRD

https://www.researchgate.net/post/is_there_a_difference_between_crystallite_size_and_particle_size

scherrer formula valid in range of 10~20 nm range also, coherent scattering reveals  ordered internal structure in terms of homogeneity of constituents of substance, and homogeneity of grain size and their distribution throughout the sample