At least in the U.S., it appears that most series which were multiplicatively adjusted in X11 are now log additively adjusted in X12.
The log additive decomposition has log X = log C + log S + log I, while the multiplicative has X=C x S x I. The first step is to estimate C. In the log additive model, that is done by taking a moving average of log C, which is subtracted from log X to get the preliminary estimate of log S + log I. In the multiplicative model, that is done by taking a moving average of C itself, which is then divided into X to get the preliminary estimation of S x I. The mean of the log is systematically different than the log of the mean which will carry through to the rest of the analysis.