Turns on/off the use of the update norm (eq. 4.42) as a way of determining convergence. The default is on. This should be turned off if it believed that the update norm is not a reasonable convergence measure.
Turning this on is useful if the behavior of the residual indicates
numerical truncation error (i.e. if the residual reduces rapidly,
but then bottoms out at a level insufficient to satisfy the residual
convergence tests). This behavior can show up with very small time
steps (TR/BDF2 and the concentration form of the generalized
trapezoid family subtract nearly equal vectors that are proportional
to 
) and nearly equilibrium reactive-diffusive systems.