# Concepts Each concept page explains one idea behind how rietx refines. Its title begins “About …”, which the menu leaves out, since the tab already says it. A page here that makes a physics claim links the Theory section that derives it. - [About a fit](fit.html.md) follows one call to `Refinement.fit` from the pattern and models to the result and the history. - [About parameters](parameters.html.md) groups the parameters by what they belong to, and covers the background and humps. - [About the report](report.html.md) describes the report’s three layers and what each can claim. - [About correlations and esds](correlations.html.md) says why parameters correlate and why some get no esd. - [About refinement plans](plans.html.md) says why a fit runs in stages, what each preset is for, and where the order they free parameters in comes from. - [About the project](project.html.md) says what each file in a `.rex` directory is for. - [About the calls most fits use](usage.html.md) ranks the API by how often the tutorials and examples call it.