Purpose. If the particle is expected to resemble a simple solid (ellipsoid, cylinder, …), fitting analytical form factors is faster and more interpretable than free ab initio shapes. Comparing several body types ranks which primitive best matches I(q).
Engine. ATSAS bodies. Autosaxs orchestrates which shapes to run and how results are exported for liveview.
What autosaxs adds.
--first from the Guinier interval when you do not override it, so the fit starts
where the low-q region is trusted.This is mostly ATSAS science; the custom layer is selection, Guinier coupling, and presentation.