Function residual_tolerance 

pub fn residual_tolerance<P, E, V>(scale: V) -> V
where
    P: Policy,
    E: FloatElement,
    V: FloatVector<Element = E>,

The residual tolerance the Newton inverses stop at: a tier-dependent number of ulps of scale, which the caller sets to the size of the quantity the residual is measured in.

The floor is the forward kernel’s own rounding (a few ulp), below which the residual is noise and the loop would run to its cap for nothing.