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.
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.