The default: physically based, valid for any Newtonian fluid - and the only model for gas, heat and non-Newtonian solves. Uses the Churchill friction factor.
The Newton convergence tolerance on the relative flow change, and the iteration cap. Defaults suit almost every network.
Advanced numerics (expert)
An expert safety net - the defaults converge unaided for normal networks, so leave these alone unless a solve reports non-convergence. Damping under-relaxes the flow update once the solve is within the damping limit of the tolerance (0 = off); the relaxation factor is the fraction of each Newton step kept while damping is active (lower = heavier). Raise the status-pass cap for a large network of interacting control valves that is converging slowly. There is deliberately no convergence-model choice: control-valve switching is a discrete -state problem, not a relaxation one, so a model picker would not fix hunting.