Iteration mode is a Restoration Parameter used in the Huygens Software to control the steps taken by the iterative deconvolution methods.
Optimized mode(aka Fast mode) the iteration steps are bigger that in
Classical mode(aka HQ) mode.
The advantage of Classical mode is that the direction of its smaller steps is sure to be in the right direction; this is not always the case in Optimized mode. Fortunately, the algorithm detects if the Optimized mode hits upon a sub optimal result. If so, it switches back to the Classical mode to search for the optimum. That is why starting always with the Optimized mode is highly recommended.