In PHX 1.2 (released on 31March2011 and available to customers on April 4th, 2011) there is a new log-additive residual error model. If this option is selected the residual model that appears in the model text is observe(CObs = C * exp(CEps)), but the engines will actually implement the model produced by log transforming both sides: observe(log(CObs) = log(C) + CEps). The main advantage to this is that the engines, particularly the Lindstrom-Bates FOCE engine, often run much faster when a simple additive error model is used. Also, the FOCE approximation tends to be more accurate in this case. 
